Post author gravator support

## Logout (preview)
![selene-logout](https://f.cloud.github.com/assets/799701/342766/40dcf81e-9dc2-11e2-9192-16b22b390936.png)

## Login (preview)
![selene-login](https://f.cloud.github.com/assets/799701/342776/67ba4f68-9dc2-11e2-8e47-f8019f5435c7.png)


I put the tags part on the buttom
The @gravatar_memoized decorator very cool XD!