Add random_suite_order option to eunit command

Option takes either 'true' or a numeric seed value. If true is passed, a
random seed is generated and used. The numeric seed value is for
repeatability.

The idea here is to root out test suites that are order dependant, or
that fail in the presence of certain orderings.
Add documentation
Merge pull request #223 from rebar/adt-random-suite-order

Add random_suite_order option to eunit command