Add support for setting global image compression..

quality. Uses an IMAGE_COMPRESSION_QUALITY in the settings.py, which, if
set, will set a quality attribute in the image processing backends. Each
image processing backend then can use the quality attribute in its "save"
method.

This has been implemented in both Pillow and Wand backends. It has been
tested with image that support this setting (JPG) and images that do not
support it (GIF, PNG) and the behavior is consistent.

This is an implementation for #163.