Implementation of Guilds in Coursemology

## Intro
This is basically a hack I wrote up for Shuqun. Apologise for the terrible code/structure in this PR. This is designed to be taken down at any point of time, so we could do a re-write with better architecture as well.

## Feature
The Course Admin or Lecturer will be able to create, edit and delete Guilds. They can also assign users to these guilds. Students will be able to see an additional leaderboard, the Guild Leaderboard. Sample pic attached:

![screen shot 2014-11-15 at 5 11 44 am](https://cloud.githubusercontent.com/assets/4353853/5053504/0ac65ff6-6c86-11e4-8d6f-b7b87377b62e.png)


## Tasks
Stuff to run if merged:
`rake db:migrate` - to add the necessary database tables
`rake db:seed` - to add guilds as a default for navbar preferences
`rake db:add_guild_to_navbar_preference` - to add guilds as an option for existing courses
 
## TBC
In the spirit of things, I will do another PR to write up the tests for Guilds later on. 
this task is not necessary. we have a general task to handle this. it can detect newly added pref.