OctoPrint GPX Versions Save

• Add a couple of machines already supported by GPX to the settings dropdown (#63)
• Pick up the Mac buildable GPX 2.6.8

Fixes:

• Hopefully reduce the likelihood of a hang on cancel
• Advertise EMERGENCY_PARSER capability to OctoPrint
• Add a timeout into cancel sync
• Filename sanitization for SD filenames
• Rename the internal python module from gpx to gcodex3g

Single bugfix release:

• Fix non-ASCII SD card filename exception on connect #76

Fix: prevent GPX from passing non-ascii on to OctoPrint

• Tweaks to allow OctoPrint-GPX to run under Python 3
• Rename the gpx python module to avoid name collisions in the future
• Fix some 'None' difficulties in gpx.ini

Pick up latest fixes in the GPX submodule

A few bug fixes:

• A race condition while pressing Save in settings could reset GPX settings
• json now accepts 'None' as well as 'undefined' to remove a setting
• zero coordinate system on connect
• update the README

The new plugin and the new GPX has a bunch of bug fixes:

• Reworked/rewritten cancel behavior, disconnect/reconnect will still be necessary, but hopefully less frequently
• New GPX has standalone daemon functionality (can pretend to be a RepRap printer on another port, more on this in the next GPX release)
• Fixed many responses to be more RepRap/Marlin like
• Better .ini file handling

Fix dev branch (1.3.0+) compatibility issue (rfind is not an attribute of None)

Bug fixes:

• Serial.write should return number of characters written
• SD printing status return value should be on a line by itself
• SD cancel should clear even if we miss the cancelled status (by asking again on build status)