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.

--HG--
extra : rebase_source : b8eaa05914

missing from the PIPE_DIR definition above, the application won't start and
doesn't log any thing to the expected locations.

Also add a sleep after printing so that the io server has
written all output before exiting. It's a work around.

Add get-deps and delete-deps to commands info string and
bash completion script.

Add create command's template variable to bash completion.

Add -c/--commands option to the bash completion script.

- simplefsm - Basic shell of a gen_fsm module
 - simplesrv - Basic shell of a gen_server module
 - simplemod - Basic shell of a module along with an associated test module.