Android IAP added to plugin-x and AUTHORS

Hi @natural-law,

Can you see that this gets merged please :)

At a later point I will come back and add support for
https://github.com/onepf/OpenIAB
So that we can have full amazon and samsung IAP's also,
I hacked my own way but dropped it as this is a better solution and the licence fits :)

@nickflink
I've started to make a move on merging back by separating out the code.
I've added you to the AUTHORS list for the great work you have done on this :)

Kind Regards
Martell


@dumganhar 
I'll be updating the main project with libcurl soon, sorry for the delay I was working on this.
I'll make sure to get mac cmake in at some point also.
I've successfully built cocos2d-x with clang on windows so I'll update that PR also. :)

This is all getting very exciting, time to kick some unity butt :P