GPX 2.1 alpha/markwal -- an unofficial GPX release

This is an unofficial development release of GPX. It includes all of the changes from https://github.com/dcnewman/GPX through 031054c as well as some local changes including the following. For more detail consult the commit history.

Added minimal serial support for the windows platform
Added support for some "query" gcodes (slicers don't [can't] really use these, but controller software might)
Added a python module wrapper so gpx can be used from python
Added support for build abort via M112 (again for controllers, you don't want an M112 in your gcode file)

omachines.zip has .ini files with the steps/mm from whpthomas's last official build. Use the values from these .ini if you want the same output as those earlier builds. One way to use them is to copy them to your gpx.ini. This build as well as dcnewman's unofficial builds include corrected steps/mm (correcting MBI's calculation error).