Fixed the plane vector equation to a simpler one (only dependent on the normal)

Removed the calculation of the inverse matrix since the rotation matrix is orthogonal, therefore inverted == transposed.
Much simpler and mathematically robust.
Implemented a least squares fit of the bed equation for auto bed leveling.

The code for the LSQ solver (qr_solve) is copyrighted by John Burkardt and released under LGPL here:
http://people.sc.fsu.edu/~%20jburkardt/c_src/qr_solve/qr_solve.html
(see qr_solve.cpp for further copyright information)
Minor code and comment polishing
Accurate bed leveling: x loop inside y and zigzag motion
Merge branch 'Marlin_v1' of https://github.com/ErikZalm/Marlin into ErikZalm

Conflicts:
	Marlin/Configuration.h
	Marlin/Marlin_main.cpp
Merge pull request #673 from fsantini/ErikZalm

Improvements to the auto bed leveling feature
Merged upstream