Fixes https://github.com/pyugrid/pyugrid/issues/24 and passes PEP8.

Adds a context manager to switch tests into the proper directory for loading test files. Bonus: fixed some spacing that had used tabs and PEP8-afied test_read.py.
Yes. In Python 3 a test.support module was added to the stdlib. It's only used for building and testing Python itself. But it has some useful stuff.  Around Python 3.3 a temp_cwd context manager was added and I think it actually works well enough in Python 3.4. It does about the same thing as chdir does here. I didn't spin off another test utilities module a) in order to have minimal impact on the pyugrid package to fix this one thing. b) to do just what works, and c) as per doing what works, only factoring something out when it becomes necessary. I only provided the necessary levels of indirection in order to keep things readable and still work.