Sort the names of plugins.

Sort the names of plugins in the documentation. Upon visual inspection, also fix a number of formulas and text issues found in the manual's description of the various plugins.
Make sure the postprocessor can also work in case we use a direct solver, where pressure and velocity dofs reside in the same block.
Use a lower degree quadrature and explain why.
Merge pull request #92 from bangerth/fix-pressure-postprocessor

Fix pressure postprocessor