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)

I am really confused here. In the initial commit you've reviewed, you said we didn't want to put this logic in models and it would be better to have it in the view controller. (It was in Comment model)

I definitely agree with not having this in the view controller but I was kind of following your suggestion.

Should I move this logic to `CommentService`?