[Feature, PARK-61] - Adding datepicker

Adding accessible date picker based on the following: 

http://amsul.ca/pickadate.js/

![screen shot 2014-09-03 at 3 23 30 pm](https://cloud.githubusercontent.com/assets/756087/4129819/7c624cc8-332a-11e4-8ee6-2b100679f697.png)

CARD - https://jira.westfieldlabs.com/browse/PARK-61?filter=-1
The majority of it is 3rd party, but I've tried to optimise in places. For example, I've done some general re-factoring, added out existing colour variables etc. @chris-pearce