Added setImageOpacity to Imagick create method

Is only called if Imagemagick version > 6.3.0
Fix #320: ImagineInterface::open now accepts any string castable object as argument

This revert compatibility with SplFileInfo objects.
Merge pull request #328 from usemarkup/develop

Added setImageOpacity to Imagick create method
Factorize imagick version extraction
Add docblocks
Add unit test to demonstrate #331
fixed pixel to color method for non-RGB palette
fixed pixel to color method for Grayscale palette
Merge pull request #326 from romainneutron/fix-320

Fix #320: ImagineInterface::open now accepts any string castable object as argument
Merge pull request #333 from romainneutron/pCoLaSD-331

fixed pixel to color method for non-RGB palette
Update changelog
Update documentation
Update branch alias to 0.7
Prepare version 0.6.0
Merge branch 'master' into develop
fix invalid namespace
Merge branch 'csanquer-fix-336'
Merge branch 'master' into develop
Add test for #336
Update changelog