Tetrahedron's top vert can be placed at centroid

Also changed the tetrahedron brush's vertices so that it points up
rather than down.
Zero value keys in transform dialog set to 1 if in scale mode
Refactored some brush script code
Merge remote-tracking branch 'upstream/master'
Further refactoring of brush script code
Merge pull request #180 from KonstantinUb/master

Added the option for tetrahedron brushes to have their peak vertex aligned with the base's centroid