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.

Feel free to ask what a particular column is for (although I have left notes in the doc.) We could also discuss whether a particular column makes sense.

Here is a preview of the schema (PK is Primary Key. Text in bold is required.)

![DB Schema](https://cloud.githubusercontent.com/assets/1449512/3085547/c88ec514-e513-11e3-8fd2-bace2d45edec.png)

Updates decided for the next iteration:

Add support for adding anonymous ideas (without registration/login) - could be achieved by creating a user named "Anon"?

Upvotes/Downvotes to comments.

Github profile url for a user.