software uart

Needs a lot of cleaning up still, though this works with the example code in https://github.com/tessel/gps-a2235h/pull/11

This PR does the following:
* Moves the uptime timer from Timer1 to Timer 3
* Timer 1 is now used for sw uart on port C
* adds hardcoded 9600 rx and 115200 tx
* adds nmea parsing code

Not part of this PR:
* multiple bitrates
* sw uart for GPIO port
* usable sw uart bindings to JS
All lua numbers are cast as `double`, not sure what gps_get_speed returns but you might explicitly cast it here.