Automatically create category and forum if either does not exist. 

This pull request...

 * updates to ZenDesk API v2 (entries are now called topics)
 * adds the `ZenDesk#find_or_create_category` method
 * adds the `ZenDesk#find_or_create_forum` method
 * uses the `#find_or_create_forum` method when creating a new topic