Assimp Importer

ENH: Added Assimp reader for OBJ and DAE files

This commit helps with the long load time in #128 (closed). It's faster for OBJ now, but still pretty slow. At least now you can choose another format like DAE, which is faster and more powerful than OBJ. This commit also addresses #78 (closed).

CLEANUP: Fixed naming of getVerticeNormal to getVertexNormal in imstkSurfaceMesh

This wasn't an issue, but I fixed it anyway. It's a very simple fix since nothing currently uses it.

ENH: Added tangents and bitangents to SurfaceMesh, and added ability to set normals

This commit somewhat addresses #133 (closed) by at least providing the data. It still won't work in VTK.