Add links to subscribe to our blog by email and feed

Adds a subscribe link to email, and to our feed, at the top of the page. It looks like this:

![news subscribe](https://cloud.githubusercontent.com/assets/4592/5210379/85df9fec-7599-11e4-9421-169ef4ce918b.png)

Fixes #371. Thanks to @gbinal for pointing this out.