Fixed issue #36 (setFirmwareVersion leaks memory)

This pull request includes a new method (`unsetFirmwareVersion()`) on `FirmataClass`. This exists only to ensure that the unit test covering the memory leak fix is independent of other unit tests.

An alternative approach, whereby a new FirmataClass instance was created within the test, failed due to the amount of SRAM this required (reporting of unit test results via the Serial port got corrupted, presumably due to stack/heap collision).