Clarify LGPLv3+
Merge pull request #8 from ralphbean/master

Clarify License
This is enough for testing that surface.cairo is a cairo Surface.
Use cairocffi when available instead of pycairo.
Use cairo instead of pysticia to load PNG test results.
tox.ini: run tests with cairocffi, test on Python 3.3 and PyPy.
Write output pngs generated by tests
Fix a context.save/restore mismatch
Draw nested tspans
Don't remove trailing whitespaces of tspan text before tspan child