Add all of the files that describe the Davies et al. benchmarks.
Cleanup some of the merge problems with this patch by undoing some changes.

Undo accidental changes originating in a botched merge on Joey's branch.

Remove the free surface description. This piece needs to move to a different
branch and pull request.
Add forgotten files.
Rename the directory for a plugin.
Edit the plugin files to match our usual standards:

Reindent and minor edits. Properly declare that the viscosity depends on the temperature.

Merge the .h and .cc files for this plugin.
Go through the new section and edit and revise it. Make the input files in the doc/ directory only those pieces that really matter.
Finish editing the manual:

Slight edits in explaining the process of loading benchmark shared libs.

Minor edits.

Reference a particular section.

Document some of the results.
Add an entry.
Merge pull request #190 from bangerth/WDurkin-davies-benchmarks-2

This patch has all of the files that are required to reproduce the first few of the Davies et al. benchmarks, plus a section in the manual that describes these benchmarks.