Warning handled for Python3: DeprecationWarning: tostring() is deprecated. Use tobytes() instead.
Merge pull request #24 from subbyte/master

Warning handled for Python3: DeprecationWarning: tostring()
Catch both IndexError and TypeError (tests now pass with numpy).
Reflow docstrings to 72 column limit and fix some minor PEP-8
violations.
Move png tests into test_png.py and the related PngSuite into
pngsuite.py.
Remove prefix from Error classes.
Removing commented out code.
Move check_color out into top-level function.
minor edit joining not-so-long lines.
abstract into check_sizes() function.
Move bitdepth/colortype check into its own function, and split
into more errors.
From main