Fix and test for issue #154 (dividing numbers of the same unit type)

Dividing two numbers of the same unit type now returns a value with no unit, to match ruby sass functionality.

This doesn't address the differences between pyscss and ruby sass when dividing by a number of a different unit type. The expected result for that is unclear, so should probably be left as is - ruby sass tends to raise an error when trying to do this, but they don't document or test that anywhere, so it may be a bug at their end.