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.
Would have been possible - won't change that. The VM does autoboxing/unboxing so the Integer "cast" ist just a hint to the VM, to use the correct method definition.