Improved VertexDeclaration class

Added AttributeUsage_Unused
Improved code and error handling
Added SkeletalMeshVertex struct

Alias of VertexStruct_XYZ_Normal_UV_Tangent_Skinning
Reworked SkeletalMesh class

It now uses a VertexBuffer
Reworked VertexMapper to make it way more generic

Will work with any attribute and vertex declaration
Reworked TriangleIterator

Will now output vertex indices instead of vertex data (making it way
more useful)
Reworked SubMesh methods
Improved code
Updated Skinning code