Adds a method to extract a single file from a Zip Archive

I did this after asking on SO:
http://stackoverflow.com/questions/25016006/unzipping-a-single-file-from-archive/25020622?noredirect=1#comment38927280_25020622

This could be useful if someone has to extract a single file from a big archive.
Method could be refined but works already perfectly and i made two tests also.