Add support for reseting the Eunit environment after each test

#### Overview

Add a command-line paramter, reset_after_each_eunit=[true|false] that if true runs each eunit suite individually and employs @slfritchie logic to reset the enviroment after each one.

This was done because tests within replication were found to be polluting the environment of subsequent tests.

#### Related PR
https://github.com/basho/rebar/pull/107