Run eunit suites in the order specified in the 'suites' command line parameter

Normally, Rebar runs eunit tests in the order the beam files are
stored in the file system (see rebar_utils:beams).  However, sometimes
it is desirable to run the tests in a different order (e.g. to
reproduce an error found on a build server).  For that case, it would
make sense to use the 'suites' parameter not just for selecting which
modules to consider, but also for choosing the order.
Merge pull request #24 from legoscia/master

Allow specifying eunit suite run order in 'suites'