Adds post details to the comment page header

The post object was nil for comments in CommentViewController, so a new request added to PostService to get a single post or update it. It also adds the ui elements for the header to the comment page to match the design of notification details page.

@koke May I bother you with a review since you've already reviewed most of the changes? If you can test it with a self-hosted site, that'd be great! If you can't please let me know so I can find a way to do it :)

Adds `authorAvatarURL` to `Post` model. /cc @astralbodies

/cc @jleandroperez 

Closes #2528 

Before:

![before iphone 6](https://cloud.githubusercontent.com/assets/662023/4628841/a0af7e06-539c-11e4-99cd-9e6f7d1afd46.png)
![before ipad](https://cloud.githubusercontent.com/assets/662023/4628840/a0aef88c-539c-11e4-888d-f85d0c62a72a.png)

After:

![after iphone 6](https://cloud.githubusercontent.com/assets/662023/4628852/ccafd640-539c-11e4-85c2-bfb44b4388ee.png)
![after ipad](https://cloud.githubusercontent.com/assets/662023/4628851/ccad107c-539c-11e4-98c9-103e2b618d4b.png)

If the avatar URL is `nil`, the `NSURL` initializer will crash