[#302] Add the newcategory Command

* Add the newcategory command and the createCategory API function,
  allowing the user to add a new Category to a Project.

* Add the Options command group, adding the fields, categories and
  newcategory command to it.
* Add the redmineDecode function to Redmine.Monad to decode JSON
  responses in the Redmine monad.

* Modify the fromJSON instance of the Category type so that it correctly
  parses responses with a single Category instead of a list of
  Categories.

Closes #302: Add a newcategory Command