Made post size chanagable, increased to 500.

This resolves issue #28.

The templates, JS, and forms have been made more generic. These now take the max values from the objects and ultimetley from pjuu.posts.backend.MAX_POST_SIZE.

I have also updated the About Me form and template to match.

Also fixed an issue where post count wasn't showing correctly on the profile page. This wasn't updated since the 'uid' key in Mongo became 'user_id'.