User management support. Refactoring. Cookie Auth.

* It is possible to get, query, add, edit and delete users.
* The RequestHandler supports optionally authentication with cookies instead of sessionId.
* Exceeptions occuring in ProcessRecord are now catched inside loops, so other operations
  can proceed on error.
* Executing a DELETE RestRequest will behave POST-like, not GET-like as intended by RestSharp.
  Otherwise deleting a user wouldn't work properly.
* Refactoring and small fixes.