Ember 1.9.0 release post

TODO:

* [x] Need some feedback from @rwjblue on details points
* [x] Is `beforeObserver` deprecated in 1.10?

[Read as HTML](https://github.com/mixonic/website/blob/blog-1.9/source/blog/2014-12-08-ember-1-9-0-released.md)
@kategengler can this not return?