Give ModelMeshCollection/ModelBoneCollection custom Iterators

These interators are ```struct``` so as to reduce garbage.

Implementation matches XNA as pointed out by @tomspilman
http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.graphics.modelmeshcollection.getenumerator.aspx
http://msdn.microsoft.com/en-us/library/microsoft.xna.framework.graphics.modelbonecollection.getenumerator.aspx

Fixes #1611

Files have been whitespace tidied also (as I was rewriting most of them), so you may want to use the whitespace free diff:
https://github.com/danzel/MonoGame/compare/mono:develop...danzel:fix-1611?w=1