remove relative import
improve testing infrastructure

defines a TESTDIR variable in `test_helper.py` with the directory the tests are
running in, and attempts to create a `data/` subdirectory for running tests

this will be a problem if there is no write access where the tests are
installed, so I think that in the future it would be best to remove any need to
write data to disk (perhaps using StringIO)
Merge branch 'master' of github.com:njwilson23/karta