Update pic12_executive unit tests to support 16lf1847

- modify bus_slave test to use correct pin for i2c comms
- modify bus_slave test to not check slave_handler explicitly
  so that we can get rid of it w/o breaking the test.
Update build_settings to use newly created pic16lf1847 processor
Update gpsim support to be functional on par with sim30

- now fully integrated into sim framework.
- feature parity with sim30.
Add patch for gpsim to support pic16lf1847
Update README with how to install gpsim