Update for libstatgrab 0.90

Update the Python bindings to match the libstatgrab 0.90 API, and generally be a bit more Pythonic (report errors as exceptions, remove redundant prefixes on names, have some unit tests).

This provides the old API, so in theory, it shouldn't break existing pystatgrab code.

This also changes pystatgrab's license to be consistent with libstatgrab (LGPLv2+), which fixes issue #2.