Fix GridOutCursor iteration in py3 PYTHON-310
Update write commands support PYTHON-554

This change brings write commands support up to
date with the current spec and gets tests passing
with the latest nightly builds of MongoDB 2.5.x.
Fix a few documentation issues.
Fix formatting issue in collection.py PYTHON-619
Fix rs network timeout test.
Simplify and fix interrupt test.
PYTHON-623: do not allow sending _ids with a key starting with $