[#289] Add Concurrent API Requests

This patch adds functions to the Redmine module to make concurrent API
requests and modifies hkredmine to use this functionality.

* Add the redmineMVar and redmineTakeMVar functions to Redmine.Monad
  module, allowing the forking of Redmine actions, with results stored
  in MVars.
* Add source repository information to the cabal file.
* Add the threaded flag to hkredmine's build options.

* Modify the printFields, closeIssue, startTimeTracking and
  stopTimeTracking functions in Main.Actions to make concurrent API
  requests.
* Modify the argsToIssueFilter and argsToIssueObject functions in
  Main.CLI to make concurrent API requests.

* Refactor the Redmine Monad types and functions from the Redmine.Client
  module into the Redmine.Monad module.

Closese #289: Use Concurrency When Possible