Implement retrieving custom field definitions

The attached changes utilize the custom_field-api created by http://www.redmine.org/issues/9664 to retrieve the field definitions from the redmine instance.

The existing unittest still work, two new unittests verify the new functionality.
Just to be clear: This was copied from existing code in the project - I suggest that someone fixes that in the other places too (CustomField.java would be the first).