Fix float precision issue causing pattern overflow

[joneuhauser]

Closes #7

• You shouldn't use unittouu anymore and in particular never on a bounding box, which is already in user units.
• The overrrun itself was simply a float rounding bug. Usually stretching the shape by a tiny amound already "fixed" it.
• Removed dependency on inkex.py which is deprecated since 1.0 and has been removed in 1.2.