Code style
Apply quality option only to JPEG images

The quality option is specific to JPEG and should not be applied to other formats. For instance, it could lead to wrong PNG compression level.

Also fixed a possible error (imagepng() arguments shifted) in GD with PNG if filter specified but not quality.
Add "png_compression" option

Takes an integer from 0 to 9.
Fix PNG compression option in Imagick/Gmagick

I didn't use the "png:compression-level" define, as it's not implemented in Gmagick nor in old Imagick versions.
Update PNG compression settings in some unit tests

Instead of replacing with "png_compression => 0", just remove these options to improve code readability.
Remove preliminary test to improve code readability
Rename PNG compression options, add changelog entries

* Upcoming "png_compression" renamed to "png_compression_level"
* Existing "filters" renamed to "png_compression_filter"
Implement png_compression_filter option in Imagick/Gmagick drivers
Validate png_compression_filter option
Validate png_compression_level option as well
Adjustments in documentation
Add "png_compression_level" option to the documentation

"png_compression_filter" not documented for now, to avoid cluttering
Rename "quality" option to "jpeg_quality"
Factorization of backward compatibility for save options
Merge branch 'quality-options' of github.com:vlakoff/Imagine into vlakoff-quality-options
Add comments about BC until version 1.0
Merge branch 'vlakoff-quality-options' into develop
Update changelog