1.8 blog post

A blog post to announce Ember.js 1.8. I'm sure I have forgotten significant items of note, please make edits after merge or PRs against my branch.
Ember.Set also changed iteration behavior (forEach, etc) significantly in a breaking way. However, it was marked as private but I think it's important to note both the breaking behavior and its private nature (to show our comittment to semver). Library authors using it may want to update.  We ended up writing a polyfill for Ember Data that may be useful to other library authors currently using this private API: https://github.com/emberjs/data/blob/master/packages/ember-data/lib/system/map.js