Refactor Rake task and add tests.

This is the first step in refactoring `ember-dev` to allow multi branch testing. I wanted to get this PR out for review while I am still working on the actual branch testing code.

This PR splits out some of the responsibilities from the original `test` task and adds tests for the majority of new additions.  As of right now this runs without change in Ember and Ember Data.

Feedback is welcome...