Feature request: 8192-sized atlases

[JJC1138]

Hello. I just have a very quick feature request:

Unity 5 supports 8192x8192 textures, so I'd like 2D Toolkit to have support for them too.

It looks like a verrrry easy feature to add too. I just searched for 4096 in the sources, added 8192 to allowedAtlasSizes, and changed the importer.maxTextureSize to 8192 in three places, and that seems to have worked fine!

Thanks!

[unikronsoftware]

I've added it to the codebase, will be in the next update.