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

Disclaimer, my web-programming skillz could use some work.  Suggestions welcome.
The REST purists always reminded me that PUT should be used instead of POST for idempotent operations, but I don't really have a strong opinion on that (or any sense of the practical implications of one over the other for these kinds of APIs).