Refactor group and node manipulation from H5File down into H5Group

This PR allows H5Groups to create and remove subgroups directly without requiring the user to manually dig out `_h5_file`.

It also extends the behavior of most functions creating new groups and nodes to also return those nodes.
According to this: http://svn.python.org/projects/external/Sphinx-1.0.7/doc/ext/autodoc.rst
Sphinx will get the modified docstring.