Blender Git Loki

Added upcast functions for safely converting a generic ID pointer into
one of the DNA structs.

This uses the GS macro to compare an ID block's prefix to the associated
DNA struct. The static_cast_id variant only does contains an assert,
while the dynamic_cast_id variant safely checks and returns a NULL
pointer in case the ID type doesn't match.

A new utility header for type traits has been added. These are standard
techniques used in boost and C++11, but added as own implementation here
to avoid nasty dependencies.