`ember test` with Testem.

Not ready to be merged yet, but would love feedback as we advance.

Things to do

 - [x] Support testem options
 - [x] get tests running
 - [x] Add documentation


In the future it would be great to make easy for people to extend/patch task so in the `test` case everyone could write their own on top of their favorite runner and just installed them from bower.