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.
This is an identity comparison. Equal (in sense of .equals()) Integers may be different according to == (and !=). Please, use .equals and provide proper null-handling code.