Added ieMeshInterpolation support for Houdini.

ToHoudiniPolygonsConverter will create a primitive attribute named "ieMeshInterpolation" and set it according to the MeshPrimitive::interpolation. "catmullClark" maps to "subdiv" and "linear" maps to "poly". FromHoudiniPolygonsConverter will detect the "ieMeshInterpolation" attribute and set the interpolation accordingly. The attribute name and values match IECoreMaya functionality.
Merge pull request #8 from andrewkaufman/houdiniMeshType

Added ieMeshInterpolation support for Houdini.