Add a JSON API function for logging in

Review Request #3 — Created May 20, 2007 and submitted — Latest diff uploaded

Review Board SVN (deprecated)
Added a JSON API function for logging in to Review Board. This allows the caller to get a nice JSON response code instead of calling the existing /accounts/login/ page, which would end up returning a redirect on success and an HTML page with an embedded error on failure.
Attempted a login using the new function. It appeared to work fine, as I was then able to post a review request.