Add Windows/Mac compatibility, implement the UBX-RXM-RAWX message, fix timeout bug.

[davidallen353]

This pull request includes 3 distinct changes.

1. There does not appear to be a version of the spidev module that functions natively on Mac or Windows. I added code to bypass the SPI functionality if not on a linux system. A useful warning when this bypass is triggered.
2. The core.Parse._read_until() static method will continue to read until it finds a terminator. However this method bypasses the timeout, so if the read misses the terminator, then it will continue indefinitely. I used the timeout defined by the stream input to act as a timeout for the read. If the timeout is reached before the terminator is found, a SerialException is raised.
3. I added the UBX-RXM-RAWX message that includes pseudorange, carrier phase, and doppler measurements.

These changes were tested on a F9P and M8N. (Note the M8N doesn't support RAWX)

[davidallen353]

I just looked at PR #10 and the separate NMEA/UBX parsing may make the timeout bug that I found difficult to replicate. I was seeing a bunch of NMEA messages come over the serial port when it missed the terminator, so it is possible that splitting NMEA and UBX would fix everything.

[davidallen353]

@edspark Is there anything that you need before considering this pull request?