Globalized fixes

Here are a couple more tweaks I had to make to get globalized ember-mocha to work.

* the `ember-mocha` module lists ember as a dependency, and so you get a runtime error when you try to eval ember-mocha.js, so define a module for ember.

* Mocha sometimes delegates back to itself internally. For example `it.only` invokes `it`. When running with modules, this is not a problem, since ember mocha's `it` does not override `mocha.it`. However, when running globalized, `mocha.it` is replaced, and so `mocha.it.only` will call ember-mocha `it`, and it needs to work with the return value.