This commit changes how rebar determines which apps have been
updated, added and removed from a release during appup generation.
Rather than use app files it now determines this from the rel file
in each version of the release. In addition it fixes a bug reported
on the mailing list when generating appups when an application has
been added or removed from either release.

Rebar will exit with {error,bad_directory} when trying to restore the code
path after it has finished working on a subdirectory if there are invalid
relative paths in it. The problem was seen when executing the last line of
rebar_erlc_compiler:doterl_compile/3 (true = code:set_path(CurrPath)).

This change adds support for executing ct test runs based on test
specificiations, which was missing previously. The rebar_ct module
now looks for any number of files with a name ending in `test.spec`
and if it finds one or more, passes these after the `-spec` argument
to ct_run instead of explicitly configuring the config, user config
and coverage config variables.

When no specifications are found, then the module behaves as it did
before this change, and both the ct1 and (new) ct2 integration tests
appear to show this is a backwards compatible patch.

To further support OTP releases I have added support for generating
application appup files. These include instructions that systools uses
to generate a relup file which contains the low level instructions
needed to perform a hot code upgrade. My goal with this module is to
produce "good enough" appup files or at least a skeleton to help one get
started with something more complex. If an appup file already exists for
an application this command will not attempt to create a new one.

Usage:
$ rebar generate-appups previous_release=/path/to/old/version

Generally this command will be run just before 'generate-upgrade'.

If an app uses -include_lib for its own included files, compilation
fails if the app directory isn't in $ERL_LIBS because code:lib_dir/1
will return an error. An absolute path needs to be added to code path
instead of just "ebin".

In git origin/HEAD is a pointer to the default branch.  This patch
allows two alternatives to explicitly specifying "HEAD" in git VC specs.

The first is a 2 arity form {git, Url} and the second is {git, Url, ""}
which worked in pre-update-deps rebars.

I have moved these macros from rebar_core.erl to rebar.erl
in order to prevent eunit tests from failing (which they
currently are).

To support OTP release upgrades I have added support for building
upgrade packages. Support for this is included in the
rebar_upgrade module, specifically generate_upgrade/2. It requires
one variable to be set on the command line 'previous_release' which
is the absolute path or relative path from 'rel/' to the previous
release one is upgrading from. Running an upgrade will create the
needed files, including a relup and result in a tarball containing
the upgrade being written to 'rel/'. When done it cleans up the
temporary files systools created.

Usage:
$ rebar generate-upgrade previous_release=/path/to/old/version

This also includes a dummy application that can be used to test
upgrades as well as an example.

Special thanks to Daniel Reverri, Jesper Louis Andersen and
Richard Jones for comments and patches.

This change makes it possible to type the beginning (the prefix) of a
command name and rebar will guess the full name of the command,
thereby saving the user precious keystrokes.  As long as the prefix
matches only one command, rebar runs that command, otherwise rebar
prints a list of candidate command names. The "-" character is
considered to be a word separator and the prefix matching is done per
word.

Example prefix matches:

    co       ==> compile
    cl       ==> clean
    create   ==> create
    create-a ==> create-app
    c-a      ==> create-app
    c-app    ==> create-app

On one project I have a need to specify port_sources on R14 only
and on another different project port_sources for Darwin and Linux.
To this end add support to handle tuples of the form
{ArchRegex, PortSource} in the port_sources list, eg:

    {port_sources, [{"R14", ["c_src/*.c"]}]}.