Fix dispose calculations

- use correct dispose mode
- only apply the dispose on extent of the previous frame
Handle transparency between frames for animated GIFs

Show the previous frame for transparent pixels when the disposal method is 'do not dispose'.

This fixes issue 634.
only update the disposal_method if it not 'unspecified'
More tests for ImageFont.py
Add test image using ImageFont's default font
flake8 and fix path in __main__
More tests for ImageMath
flake8 ImageMath.py
Added class checking to __eq__ function
Test case for #774
Merge pull request #1 from hugovk/775_tests

Test case for #774
Don't exclude Windows code
Document environment variable method for pip, #720 [ci skip]
Remove unused WIP save2() from pre-fork days
Merge pull request #779 from wiredfool/docs

Document environment variable method for pip
Multiplication needs to be 64bit, to handle overflow regardless of the bittedness of the machine, fixes #771#
Windows compatibility
Test for #771, hangs in nose
remove extra import
Test _util.py