Add Gmail action buttons to Devise emails

Add the necessary markup to add the inbox action buttons:

![screenshot_6_8_13_10_49_am](https://f.cloud.github.com/assets/44360/627242/99e76f04-d010-11e2-9d4a-8ef3a3118748.png)

To enable this, register your app with Google:

https://developers.google.com/gmail/schemas/registering-with-google