Enhance support for querying RT

I'm trying to use rtkit to extract information from RT, which looks to be an area it hasn't be used for much yet. This set of patches:
* Fixes authentication using CookieAuthenticator
* Implements the search_tickets method
* Adds support for custom fields and Requestors to the Ticket class

It would be lovely if you could merge these or let me know about any issues with them.

Thanks,
Alex