Bitcoin: support x btc in yyy queries

Fixes #431

![screenshot - 08302014 - 08 44 16 pm](https://cloud.githubusercontent.com/assets/4411471/4100647/1dc70106-30a8-11e4-9304-44c5178834bb.png)
![screenshot - 08302014 - 08 44 45 pm](https://cloud.githubusercontent.com/assets/4411471/4100648/1fcea03a-30a8-11e4-8370-05bf30ddf3e8.png)

I'm using the `buy` price in both cases, but I'm not really sure if that's correct (it's not my area of expertise). We might want to round the btc value to more decimal places as well.

cc @chrismorast for design.