[assimp] Implemented MAcOS support for AssimpNet

Also upgraded the libassimp.dylib to be a
universal binary.
Fixed up AssimpNet to look in the executing assembly directory
for the libassimp.dylib. Gets rounds an issue when executing
MGBC.exe on mac.
Merge pull request #31 from dellis1972/master

[assimp] Implemented MacOS support for AssimpNet