Xref not being stopped caused a crash in subsequent xref runs.

This change makes it possible to assign pre/post scripts to all
rebar commands. This allows users fine grained control over
when scripts and/or shell commands should be executed, where such
extensions are absolutely needed.

Several examples have been added to the rebar.config.sample file.

Currently with default simplenode template, `appname console other_args`
will pass ["console", "other_args"] as plain arguments. But
`appname start other_args` effectively calls `appname console`. This
patch replaces this call with `appname console other_args`.

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.

`init:get_plain_arguments()` returns `["console", "more_args"]` when
started with `$APP_NAME console more_args`, but `["console"]` when
started with `$APP_NAME start more_args`. This patch makes `start`
behave like `console` in this respect.

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".

This change adds a simple common_test suite template
that can be instantiated with the name of a module
under test like so:

`rebar create template=ctsuite testmod=mymodule`

The template creates an empty test suite in the test
directory, automatically exports test functions and
sets up a first, skipped test function.

The trailing slash for PIPE_DIR is necessary for
both start and attach operations.

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.