Manual rebase of the avoid_crossing_perimeters feature
Make the motionplanner work again after recent changes to approximation
Better implementation of the external motion planner
External motion was crossing still too much
Dynamic extrusion width for better gap filling
Merge branch 'master' into avoid-crossing-perimeters

Conflicts:
	README.markdown
	lib/Slic3r/ExPolygon.pm
	slic3r.pl
Revert "Refactoring: initialize all layers at once and avoid duplication of slicing height math". #637

This reverts commit 08270022dd9bebfb199448db3acc631e9049d133.
Add example costs calculaton. #644
Refactoring: new Slic3r::Model class to represent files
Export plates as AMF constellations
Refactor the AMF parser so that it popolates a Model object directly. Also read constellations
Unfinished work for decoupling GUI from the Print object (goal = more speed for manipulation, less memory usage)
Support legacy config files not containing first_layer_height #631
Adjust M-codes for temperature handling for Teacup. #539
Releasing 0.9.2
Bump version number
Fix MANIFEST. #656
Remove the initial retraction compensation. Not only it is not very good for multi-head setups, but it also caused wrong first layer Z when lift was enabled. #655
Consider single walls as external perimeters. #661
Use support material extruder for brim. #653