Review/Rewrite MTK3339 GPS

[CaiB]

Current implementation is lacking some important functionality. Sensor should be fully implemented, taking into account UART communication issues.

[CaiB]

I found the device datasheet here: https://cdn-shop.adafruit.com/datasheets/GlobalTop-FGPMMOPA6H-Datasheet-V0A.pdf
It appears to use standard NMEA data, documented here: http://aprs.gids.nl/nmea/
The ones the device uses are defined in the datasheet.
Commands are documented here: https://cdn-shop.adafruit.com/datasheets/PMTK_A11.pdf

[CaiB]

I'm moving this up to 0.6.0. The current implementation technically works, and there's still a decent bit of work to do on this version to get it where I'd like, so a timely 0.5.0 release is more important.