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'.

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.

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

Modify rm_rf and cp_r to work when {win32,_} = os:type().
Simplify rm_rf to only accept one filename, directoryname or wildcard.
Add unit tests to ensure a similar behaviour on windows and unix.

Thanks to tuncer for guidance and feedback.

Modules that include the EUnit header get an implicit test/0 fun,
which cover considers a runnable line, but eunit:(TestRepresentation)
never calls.  Result: prod modules with tests can never reach 100%
coverage.  Ironic.  In this case, fix it by decrementing the NotCovered
counter returned by cover:analyze/3.

1.  When running the eunit command with the convention of putting
tests in "*_tests" modules, eunit would run those tests twice.  This
is because: 1) eunit:test/1 will naturally look for foo's tests both
in foo, and in foo_tests, and 2) eunit:test/1 was being folded over
all project modules.  The fix is to filter "*_tests" modules from the
list passed to eunit:test/1.

2.  When running the eunit command with cover enabled and tests in a
'test' directory, cover would error because it couldn't find the
source code for those tests.  This is because cover:analyze/3 will
only find module source in "." and "../src".  This is hard-coded in
cover :-(.  Since cover shouldn't be calculating code coverage on test
code anyway, the fix is to not fold cover:analyze/3 over
non-production code.

3.  When running the eunit command with cover enabled and a test suite
defined, cover would only attempt to calculate coverage on the the
test suite itself.  This was because only the suite was passed to
cover:analyze/3.  The fix is to fold cover:analyze/3 over all the
production code, filtering out the suite module if it is defined.