serve command (http api)

This pull request adds a serve command which hosts a http server that listens for POST and GET requests. At the moment it's only possible to GET and POST individual blocks (blobs?).

I decided against implementing the API like it was described [in issue #2](https://github.com/jbenet/ipfs/issues/2) because
* having a /ctrl/add path is not restful since / is the location where the POSTed files end up
* distinguishing between uploaded blocks and files is unnecessary since a client can just chop the file up themselves if they want a chunked upload
I did it with an interface because it makes it easy to mock out the ipfs methods by swapping in a test interface. Another way of doing it would be to have function variables but that felt less go-ey and more like javascript. If there is a third, nicer way of mocking them out I'm of course open to suggestions.