Add Handlebars 2.0 update blog post.

Needs to be held for the updates and packages to be published as described (hopefully soon), but I wanted to get this out for review and to start letting folks know what is happening....
I suggest stating the dates.

> Unfortunately, Ember will not be able to maintain backwards compatibility to 1.x versions of Handlebars while
> supporting 2.0. In Ember 1.9.0 (which will be released to beta on October 24th and stable on December 5th)
> support for Handlebars 1.x will be removed, and support for Handlebars 2.0 introduced.