Show posts in the web interface

This pull request displays posts in the UI if they've been added via the API.

Some points of note:

- This only adds the ability to *view* posts in the UI, I haven't added any editing capabilities as that seems like a can of worms and we're generally trying to avoid confusing users with the concept of posts where possible.
- I've used the same initials method from people (first letter from the first and last word of the post label), but it doesn't really look right for posts, suggestions welcome. (See first screenshot below as an example).

Still to be done

- [ ] Add an icon to `public/img/icon-posts.gif` which represents the concept of a post

#### Posts on the homepage of an instance

![screen shot 2014-11-04 at 14 35 49](https://cloud.githubusercontent.com/assets/22996/4901291/319b63a0-6430-11e4-8f55-71914a45466a.png)

#### Individual post page

![screen shot 2014-11-04 at 14 35 58](https://cloud.githubusercontent.com/assets/22996/4901371/dd4b921a-6430-11e4-92b3-761851a24b63.png)


Fixes #653