Update Oauth authorization view - nicer styles

Make the oauth authorization view look a bit nicer.

Add user avatar and added a few styles. 

![image](https://cloud.githubusercontent.com/assets/494927/4442323/1ce5ddbe-47de-11e4-8ecb-765e17091845.png)

TODO - add info on 'scopes' - I added some styles for this but as its not implemented yet I have removed any HTML.

Taken some inspiration from the 'connect with Jetpack' auth screen - see example here: https://wp-stream.com/