Merge pull request #586 from xoppa/3dapi

Add GLTexture and Cubemap
made GLTexture abstract, fixed up a few minor things
fixed texture emu in gwt
added notes about texture changes
removed texture emu, replaced with ETC1TextureData emu, nice :)
Fix Label docs

Doc stated a Label would honor actor rotation/scaling but that functionality was removed long ago.
fixed pixmap disposing in texture data upload
Merge branch 'master' of https://github.com/libgdx/libgdx
Update README.md
Update README.md
Update README.md
Cubemap add javadocs