Fixed test-parseNavdata tests (issue #72).

* Removed the failing zimmu3000 option test--I think it's been obsoleted
by GPS.

* Fixed up test/fixtures/navdata.bin by replacing what looked like a bogus GPS
option with a real one, recorded from firmware 2.4.7 (and updated the checksum
of the entire data packet).
Added unit tests for gps option.

Not all values look legit to me--longitude, hdop and vdop look suspicious.

I changed droneTimeToMilliseconds so it won't return negative times and
fixed gps.last_frame_timestamp to be in milliseconds.
Merge pull request #73 from wiseman/fix_tests

Fixed parse-Navdata tests and added tests for gps.