0.12 rc1 blog post

I haven't cut the builds or tagged the repo or any of that but getting this prepped. Mostly written by @sebmarkbage and then I wrote more/edited.
![screen shot 2014-10-16 at 13 32 50](https://cloud.githubusercontent.com/assets/8445/4670064/a8f8030c-5573-11e4-8d0d-9f64cbeba910.png)

s/custom-elements/custom elements/