Fix #327: support for inplace binary operators

(still need to add more tests)
More tests, and fix integer floor division
Add more tests
Refactor tests to use generation instead of copy/paste
Add tests for complex numbers and arrays
Remove unused code and fix typos
Exercise object mode more thoroughly by using force_pyobject
Merge pull request #617 from pitrou/inplace_binops

Correctly implement inplace binary operators (for mutable types)