gcodeParser: add bbox for model envelope
improve layer detection: first extrusion in a different Z

yagv: restructure code
add Ctrl-R to reload model
improve model centering
Add support for relative coordinates in G1 with G90, G91. Courtesy of https://github.com/leemes
Merge branch 'leemes' of git://github.com/leemes/yagv
update version and README