Ashtech GNSS receiver driver

Added driver for Ashtech GNSS receiver with 20Hz position update rate.

Ashtech driver will increase fix_type (visible in sdlog or QGC) to 4, 5 or 6 depending on RTCM/RTK differential status.

Tested on Ashtech MB100 board. 

The following logfile was recorded during flight without RTK diff corrections, only RTCM differential (SBAS) was used, so eph was about 0.5.
https://www.dropbox.com/s/mj5pssn6isxvb25/15_21_15.bin?dl=0

The following logfile was recorded during flight with RTK diff corrections and eph was about 0.02
https://www.dropbox.com/s/cj4m9sap6bl94l3/09_43_12.bin?dl=0


Yes, that's a successful return, but as far as I understood from gps.cpp it's more like a number of messages parsed, which will be returned by GPS_Helper::receive() and checked as 
```C
while ((helper_ret = _Helper->receive(TIMEOUT_5HZ)) > 0 && !_task_should_exit) {
```
in src/drivers/gps/gps.cpp, line 362

I basically tried to do the same stuff as in ubx.cpp