Better support for LA mode images in PIL engine

This allows LA mode images (i.e. greyscale with transparency) to preserve their transparency when using the RGB colorspace.
Merge pull request #265 from jphalip/master

Better support for LA mode images in PIL engine
Keeps icc_profile while ignores other uneeded options in PIL engine
Merge pull request #274 from caioariede/issue_268

Keeps icc_profile while ignores other uneeded options in PIL engine. Solves issue #268
Merge branch 'master' of github.com:mariocesar/sorl-thumbnail into filters