Introduce randomization iterator

[trentmwillis]

We should introduce a new command that runs the tests in random orderings for a certain number of iterations. This would be useful as an offline job that helps verify the stability of the given test suite.

Currently this can be approximated by building the app once and then pointing to the output as the source for the test. In pseudo-code:

ember build
for 0...N
  ember exam --path dist --split 3 --parallel --random

We should set up a simple way to invoke this (maybe ember exam:iterate <num> <pass-through options>) and report all test failures at the end.

Should probably look into seeing if we can leverage/learn from ember-try for this, as they already run test suites multiple times.

[trentmwillis]

Implemented via #56.