Change the recommended versions of PETSc and petsc4py to be 3.3. This is because
I encountered a bug when I used the latest dev versions.
Restructuring Information for developers section to reduce the number of levels
more doc editing
Minor typo and style fixes
Merge pull request #233 from amal-ghamdi/doc_restructure

Restructuring the section "Working with PyClaw source code" of the documentation