[BUGFIX] Performance Improvements

[offirgolan]

Resolves #381.

Changes proposed:

• Use defineProperty instead of creating classes via Ember.Object.extend

Finds:

• Initially when I ran the test case provided in Validations slow performance #381 by @jakesjews, I got a resulting time of ~550ms. With the added changes, I now see around a 150ms to 200ms decrease in time (sometimes more).
• After a bunch of time debugging, it looks like using extend to create new classes take a lot longer than actually using defineProperty on init.

cc @stefanpenner @rwjblue have you guys noticed this sort of thing in the past? I mean... it makes sense but from past experience, defineProperty is a pretty heavy hitter in terms of performance.