Added function documentation.

Added
=====

* More documentation for calculate_steepest_descent_across_cell_faces in
  the numpydoc style.
Merge branch 'master' of git://github.com/landlab/landlab into fix-raster_funcs-docs

Conflicts:
	landlab/grid/raster_funcs.py
Added function documentation.

Added
=====

* Added numpydoc-style docstrings for:
  - calculate_flux_divergence_at_nodes
  - node_id_of_cell_corner
Merge pull request #13 from mcflugen/fix-raster_funcs-docs

Fix raster funcs docs