Segmentation: Added convenience meta functions for segment/segmentation type retrieval. Fixes #41.

The following metafunctions were added:
  result_of::mesh<SegmentationT>::type      (mesh type from segmentation)
  result_of::mesh<SegmentT>::type           (mesh type from segment handle)
  result_of::segmentation<SegmentT>::type   (segmentation from segment handle)