Fix issues #119 and #132 and implement #163

Concerning #163, an IMAGE_COMPRESSION_QUALITY settings can be set which 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.