Added eight point BoundingSphereTest

Added eight point BoundingSphereTest that works with XNA test
environment but fails with monogame test environment
fix BoundingSphere.CreateFromPoints
Merge branch 'develop' of https://github.com/stampergames/MonoGame into BoundingSphere
Merge pull request #3162 from nkast/BoundingSphere

fix BoundingSphere.CreateFromPoints
avoid Sqrt , move code to ref Methods
Fix the Xamarin Studio Content Pipeline addin to build with the correct dlls

This is because the paths to the monodevleop dlls changed in the
latest release
Merge pull request #3166 from dellis1972/fixmacbuild

Fix the Xamarin Studio Content Pipeline addin to build with the correct dlls
Merge remote-tracking branch 'upstream/develop' into optimizeBoundingSphere
Merge branch 'develop' into xamarin
Merge pull request #3116 from mono/xamarin

iOS 8: Handling of portrait and hybrid portrait/landscape
Merge pull request #3163 from nkast/optimizeBoundingSphere

Optimize BoundingSphere
Merge branch 'develop' of https://github.com/ebatiano/MonoGame into develop