(Issue #30) pymaging.image.flip_left_right should not flip image vertically.

pymaging.pixelarray.copy_flipped_left_right no longer transposes the
rows of an image.

Also fixed offset/index error in pymaging.pixelarray.copy_flipped_top_bottom
where the start position for the source image did not account for the
row length.
Color memory optimization
Color hash speedup (x5)
Color default alpha
Fix division in colors (can be used in other modules).
some colors tests (sorry, not complete)
Merge pull request #33 from homm/master

Some colors improvements
Merge pull request #32 from cgroner/issue30

(Issue #30) pymaging.image.flip_left_right should not flip image vertically
use abc for abstract class
remove unused variable
Made pymaging.image.Image.new faster

Closes #28
Implemented a delayed loading API

* pymaging.image.Image now takes a loader callable instead of a pixel array
* Added pymaging.image.LoadedImage which behaves similar to pymaging.image.Image used to
* Added pymaging.test_utils.pixel_array_factory
* Updated docs and tests
Merge pull request #35 from ojii/delayed-loading

Implemented a delayed loading API
Fixed tests failing on 2.6

Fixes #36
Merge branch 'master' of github.com:ojii/pymaging into delayed-loading