add partial support for Stage3D API

Hi,

First of all thanks for NME. This is a great lib.

Recently I have been doing some partial implementations of Stage3d on NME and I though it would be good to have it integrated into nme as soon as possible so that work do not get duplicated.

There is probably lots of stuff that I did wrong here (code style and standard for duplicating API across target...)  but since the API is already defined (Stage3D) I expect it should not be too bad :)

I have created a post here http://www.wighawag.com/blog/2012/12/NME-Stage3D-support which explains some of the current issues.

There is a test app here https://github.com/wighawag/NMEStage3DTest

Thanks again for all your work

looking forward to collaborate

Wighawag