The order in which modules, within an application, are loaded can be
important. This patch adds allows the specification of module
dependencies such that generate .appup/.relup scripts will load a
module's dependent modules before itself.

To use:

in rebar.config, add a module_deps

{module_deps, [{ModuleName, [DependentModuleName, ...]}]}.

ModuleName is the name of any module, followed by a list of module
names that it depends on.

* allow plugins to print help message for implemented commands
* append core rebar.config options to common 'rebar help' message

The target_dir config in reltool allows you to put your release in
a directory other than in ./NAME, so we should look in the parent
directory of that to find the new and old versions instead of
simply looking in ./

Move untaring and retaring into a temporary path instead of in ./
to prevent name collisions with "releases" and "lib" that might
exist already. Having a subdirectory rel/releases/ can be useful.

get_apps/3 now returns which apps have been added, removed and ugpgraded
in a reasonable way. It should prove more usable should we want to
access any of those lists in future appup related changes.

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.

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'.