Adds ajax router to handle ajax requests (and other things)

This PR does a variety of things:

* adds a layer of abstraction on top of ajax requests
    - verifies nonce of action (defaults to true)
    - verifies the capability (defaults to manage_options)
    - returns a standardized error format (and sends non-200 errors if necessary [for backbone])
    - handles both php://input and $_REQUEST data
* adds a better page for disconnecting your clef account
*