Add crypto contributions page to /wallets

![image](https://f.cloud.github.com/assets/750477/2405471/4315c4b6-aa51-11e3-9ae1-e7bfc44de663.png)

To be linked to from checkout page of love.loomio.org, in case someone wants to give us bitcoins instead of (inferior) actual money.
e.g. 
![image](https://f.cloud.github.com/assets/2665886/2405712/e575e442-aa57-11e3-9e12-1f4fab1408fa.png)