Gallery rearrangement

changed Gallery link at top of pages and added page galleries.rst with links to all galleries, 

Plus modification of pyclaw title.  

Could not build all the pyclaw examples and pyclaw gallery links to README and _plots are broken.  I think they were previously too.

I made some changes to gallery.py previously when adapting it from doc/pyclaw/gallery to doc/gallery and the other apps have links that work (classic, amrclaw, geoclaw).  Similar changes may be needed in the pyclaw version.