Implement M226 - GCode Initiated Pause

Implemented M226 as described here:
http://reprap.org/wiki/G-code#M226:_Gcode_Initiated_Pause

Waits for pin to be become either HIGH, LOW or the inverse of what it
was before. Allows printing to pause until user interaction
Merge pull request #647 from shaggythesheep/newfeature-M226

Implement M226 - GCode Initiated Pause