Grbl-specific driver

Pass correct type to laser property (fixes #45)

Add configurable S parameter for 100% power. 

Add a configurable spindle value to use for 100% power. Marlin uses 100, others use 1.0, stock Grbl use 1000, some use 255.

Simplify process of converting rasterized images into equivalent gcode

Implement file export as a gcode job upload method

Useful for saving to an usb-mounted smoothieboard or an sd card, instead
of manually saving to a fixed path each time.

Better error handling during serial connection errors

Add configurable option to blank laser during G0 rapid moves.

Add configurable option to blank laser during G0 rapid moves. Fixes issues in PR #38 for Smoothie boards

Turn laser off when doing G0 rapids

Fix power command missing on subsequent gcode exports after initial export

Fix Smoothieboard not responding when connected via serial

MakeBlock XYPlotter driver

Export gcode