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.