Make symbol rank optional.
Support -=, --, *=, /=, and implict casting to Symbol
None is not iterable
Fix cases in which children were not nodes.
working FunCall
Make semicolon handling for incr and decr consistent.

I am not convinced this is the Right Thing. It comes down to the statement/expression duality in C.
Make For a bit smarter about its payload
++a not a++, space around operators, handle = 0 correctly.
Documentation fixes
Merge pull request #343 from OP2/remove-cgen

Cleanup and extension of PyOP2 AST.

- Adds -=, *=, /= and ?:
- Simplifies user syntax (strings, numbers cast to symbols).