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'
Added class checking to __eq__ function
Test case for #774
Merge pull request #1 from hugovk/775_tests

Test case for #774
Remove unused WIP save2() from pre-fork days
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
Merge pull request #775 from radarhere/master

Added class checking to Image __eq__ function
Update CHANGES.rst [CI skip]
Merge pull request #782 from wiredfool/32bit_decompression

32bit mult overflow fix
Update CHANGES.rst [CI skip]
Remove unused tearDownModule
Renamed to avoid test runner
Guard to prevent 64 bit machines from crashing
Merge pull request #785 from hugovk/remove_tearDownModule

Remove unused tearDownModule()
Merge pull request #780 from hugovk/kill_save2

Remove unused WIP save2() from pre-fork days