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
let's make this command either `serve-http` or just do `serve http` (subcommand), as `serve` is not protocol ambiguous. (I can imagine `ipfs serve git` and so on in the future)