Does not ensure Ember references

[danfinlay]

Currently in several of its scripts, ember-watson will insert Ember calls, but without ensuring Ember is imported to the current file.

I ran into this because I was using a paradigm (I swear I've seen others do it) where I would import Em from 'ember', giving myself a simpler Ember reference.

Bonus points if watson can use the same shorthand that ember is already being imported as ;)