Preliminary GPS support
Missing comma
Fixed errors in GPS data definitions.

* Fixed incorrect order of unk0 & data_available fields.
* Fixed misspellings.
* Many fields are actually arrays (this was causing a lot of fields to
have nonsense values).

Based on https://github.com/paparazzi/paparazzi/blob/55e3d9d79119f81ed0b11a59487280becf13cf40/sw/airborne/boards/ardrone/at_com.h
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.
0.2.1
Added handling of ACK for config commands.

We now have two classes of commands: blocking and non-blocking.
`CONFIG` is blocking, all others are non-blocking.  When a blocking
command is sent, no other blocking commands will be sent until an ACK
is received (and cleared) from the AR.Drone.