Adding in requirements file for virtualenv

I've setup the a requirements.txt file for use with virtualenv. This will become important if we're ever going to get this live.

[Take a look at this tutorial on virtualenv](http://www.dabapps.com/blog/introduction-to-pip-and-virtualenv-python/)

Basically this ensure that we have specific versions of all the packages so we don't need to worry about versions (that tutorial and others online will explain it pretty well).