Add some documentation.
Add another reference.
Merge pull request #147 from bangerth/doc-update

ADd documentation to the hp::DoFHandler class.
Updated documentation for step-20 Introduction section and a typo in the documentation for iterative_inverse.h. Detailed: In the introduction of the step-20 tutorial, Sec. Solving using the Schur complement, the IterativeInverse class is introduced and then in the following code section not used. Now the InverseMatrix is changed to IterativeInverse< .. > to match with the code of the tutorial, as used below.
Merge pull request #149 from koecher/master

Updated documentation
Remove references to MGDoFHandler.

MGDoFHandler has been deprecated for a while already. Remove all
references to it from this documentation module.
Remove another reference to MGDoFHandler.
Merge pull request #150 from bangerth/doc-update-2

Remove a few references to MGDoFHandler as that class is no longer relevant.