Add bitcoin payment support

[bwhite]

@amiller will create a bitcoin service that accepts payments and tells Picarus when a user has applied a new credit. I'm going to use that info to update the user's balance and tie that in with resource allocation. That includes adding additional EC2 spot instances for computation (that works great with our Hadoop setup). We are going to put off actually charging for resources as long as possible on our public clusters; however, by design you can always just start your own relatively easily on your own cluster (EC2 or otherwise). This is a step towards our goal of allowing users to be entirely anonymous. Keep in mind that bitcoin doesn't provide anonymity in general, but there are ways of using it that do.