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
Instead of query parameters we should leverage http headers as much as possible. E.g:
`Accept: application/json` instead of ?format=json

And perhaps instead of query strings for data and links e.g. `GET /<path>;links` could be used, [since it is specific to a path segment.](https://restful-api-design.readthedocs.org/en/latest/urls.html#variants) It looks prettier as well. :)