100% test coverage for Jam_Range with fixes to existing functionality

Changes to Jam_Range:
 - Jam_Range::sum now accepts format
 - Jam_Range::offsetGet now uses Jam_Range::offsetExists
 - Jam_Range::serialize does not take arguments
 - Jam_Range::add now uses format of $this for the new Jam_Range object

More organized tests with @covers directive and 100% test coverage.