Allow user to input custom text for voting

NB: Not quite ready for QA / merge yet.

This allows input of custom text for vote actions, designed for proposals where 'yes', 'no', 'abstain', etc might mean something a bit more specific (for example, 'yes' could be 'I have read the minutes for this week', or 'no' could be 'I am unable to attend this weekend')

Design / copy input welcome!

Screenshots!
![image](https://cloud.githubusercontent.com/assets/750477/4181391/086ae05e-3718-11e4-8835-b9a28078fb8d.png)
![image](https://cloud.githubusercontent.com/assets/750477/4181379/e1ef87ea-3717-11e4-961e-43202dc3304a.png)
![image](https://cloud.githubusercontent.com/assets/750477/4181401/4254f5fc-3718-11e4-97ff-68d0c1a6ada1.png)
[sorry about half-faded tooltip; you get the idea though.]

@rdbartlett @s01us @jonlemmon @alannaloomio @robguthrie