For now, this is just a proof of concept; would make sense to add a lot
of things, such as quick access to invoking rebar itself (with approproate 
code reloading), as well as an eunit-aware version so that tests could be 
run interactively.

version info

required for building on a different machine to what your live
config is kept on. This way you can build on one machine,
distribute to new machine, then run overlays against a live
config to prevent accidently running code against a live
config in a development or ci environment.

Currently the --config switch does not work because when loading
a new rebar config the global setting is ignored for all paths.
This patch provides a check when loading new rebar config to see
whether or not the current config path matches the `base_dir` set in
global conf, which produces the expected behaviour.

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'.

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