Configuring Additional Case Columns - For use with third-party plugins?

The FogBugz API supports querying and updating columns / fields that are associated with other third-party plugins, as well as the core FogBugz fields associated with each case. See the following link:

http://fogbugz.stackexchange.com/questions/729/access-custom-fields-or-other-plugin-field-such-as-backlog-on-a-case-using-the

This change supports adding an additional configuration key to the fbapi4j configuration file called "case.columns". This setting (a CSV of column names) instructs the CaseHandler on which columns to request whilst performing HTTP GET calls.

Appropriate test cases added.