Send along an identifying User-Agent header with all requests

[mattrobenolt]

Something as simple as User-Agent: database-js/0.1.0 seems like it'd be sufficient. Happy to accommodate more data as you see fit, but at least we can start identifying client version.

This might require an addition to our CORS policy? So might require a tiny bit of experimentation to make sure it'll work.

Open to other suggestions too. We could potentially use a different header entirely Client: database-js/0.1.0, etc. We own everything here, so we can accommodate what works.