Ignore *.pyc files
test: add load_dng() tests

Add a basic test framework, runnable with nose.  Add basic tests for
load_dng(), which load a sample test image, checking the CFA, and data
against a known reference.

References #366
Update save_dng() docstring

Note that the ndarray must be contiguous; ValueError could be raised for
an inappropriate color_matrix.
tests: add save_dng() unit tests

Add a dozen or so unit tests for tiffutils.save_dng(), testing all of
the arguments, and several bad inputs.

References #366
Raise ValueError instead of TypeError for ndarray of wrong format

If array isn't contiguous, or has the wrong dimensions, etc, raise
ValueError.