Fixed XNA projects.
Keyable draw order for spine-csharp.
Additive blending for spine-csharp and spine-xna.
Bounding boxes for spine-csharp.
Minor updates.
License update.

Changed from New BSD to a custom license. The new license requires a Spine license to use the code. If you have a valid Spine license, you can do whatever you like with the code. This should not be a problem for anyone using the runtimes with Spine, nothing changes. If using the runtimes without a Spine license, you now need a Spine license. This is because the runtimes were created explicitly to be used with Spine.
Event timeline fixed to fire events correctly for edge cases.
SkeletonBounds refactored so state is not stored in BoundingBoxAttachment.
updateVertices -> updateWorldVertices to match bounding boxes.
Support for rendering via PolygonSpriteBatch.
Merge branch 'master' into monogame