Added a database schema

The schema currently contains three tables: users, ideas, comments.

To use the schema doc, Install this piece of software - http://oxygene.sk/lukas/projects/dbmodel/

I have added a few columns that I find are necessary. Comment a column name and the values it could contain. For e.g. I think by having the 'last_login' column we can tell the user when their account was last logged in so that fraudulent logins can be detected.

![DB Schema](https://cloud.githubusercontent.com/assets/1449512/3085547/c88ec514-e513-11e3-8fd2-bace2d45edec.png)