Deprecation Cleanup

This pull requests adds documentation:

* API.md: An outline of the API on the server as it currently stands. It gives particular emphasis to CSRF protection and establishing sessions. The target audience is developers of privly-applications.

Removes deprecated functionality, including:
* The `.iframe` MIME since it has been completely replaced by the privly-applications bundle.
* The domain redirection functionality. This was a temporary patch while we moved the priv.ly domain away from the content server.

And updates the "CDN" copy of the privly.js content script. This is essentially a hosted version of the script that I occasionally use with scriptlets. I don't recommend people use privly.js as a scriptlet but it is convenient at times.