Added a GetAtlas method

This will allow several SpineEntites share the same Atlas, if the next ones are loaded with the same Atlas the first one was created. This can make it easier to get a single texture for several characters, by creating the new ones line New SpineEntity("another_skeleton.json", PreviousEntity.GetAtlas())
I find it useful so I supose others may find it useful too.
Merge pull request #12 from ziggybcn/patch-1

Added a GetAtlas method, will inspect this and make sure the Free() method wont release atlas