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

Add generate-upgrade and previous_release=.

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"]}]}.

Add flags to rebar_utils:sh to control output and error handling.
Replace calls to os:cmd with calls to rebar_utils:sh.