Remove dependency on numpy

Closes #17.

The dependency was merely a convenience and functionally not necessary. By
removing it we gain pypy compatibility and make the package more light-weight
in general.
Fix implementation of PolarGrid2D

The breakdown of this class under the transformation was due to missing
regression tests, which have been added now. Its behaviour has been documented,
too.

To run the test suite on Python 2.6, it was necessary to hack a monkeypatching
mechanism for absent assertions in `nose.tools`.
Merge pull request #26 from aepsil0n/feature/rm_numpy

Remove dependency on numpy