add klass library

Add klass library to photoswipe. 

This lib is required for photoswipe. The release versions are not the right. They are other versions which don't run. I try to release a right version (see https://github.com/ded/klass/issues/19) but without success.

This library is deliverd with photoswipe 3.0.5.

Please add this library to the photoswipe.