Updating the PyCon API.

Note: This is a BACKWARDS INCOMPATIBLE commit.

This commit updates the PyCon API drastically:

  * It alters the URLs
      * The API key is now sent as a header, rather than part of the
URL itself.
      * URLs now respond to GET and POST, with POST being intended for
writing, and GET for reading.
  * Requests to the API must be **signed** using a secret key that is
issued as part of an API key.
  * The proposal detail endpoint actually shows details from proposals,
rather than only what was set using the API. Arbitrary data can still
be set and comes down under "extra".