Blaze server

Here is a tiny replacement for catalog.  It doesn't do any of the directory management tasks but it does cleanly expose `getitem` over a simple HTTP API.  This API takes structured JSON data and so should be easy to interact with.  Because this interacts with the `blaze.data` getitem syntax this should extend to a variety of data formats.

cc @mwiebe
@hhuuggoo can I get your impression on this test?  Is this valuable to Bokeh?

@quasiben it'd be good to get your thoughts as well.