unzip: add function unzOpenBuffer

Hey @nmoinvaz,

Thanks for merging all the patches on the mailing list. :)
Really picking up the slack :D

I work on the opensource cocos2d-x game engine and minizip is a depend.
Your repo will really help as the current version is like a miss mash of patches 
with random custom namespaces that someone decided to add.

Any way I want the team to switch to using your repo as the upstream as it is the most up to date 
but this function is needed by the game engine to support newer features.

Could you give me some feedback and hopefully we could possible merge this?