Exhaust cursor tests.

Port PYTHON-736 tests from PyMongo to Motor.
Longer timeouts in some tests.
Update PyMongo's git hash to the latest.

This is just short of PyMongo's 2.8rc0 release, includes a test fix for the latest
MongoDB 2.8 replica set behavior.
Remove link to example application "Chirp", I no longer maintain it.
MOTOR-48 Add "How to Ask for Help" section to Github readme.
MOTOR-37 Delete tox.ini.

It can't represent the matrix of multiple Tornado versions and other dependencies,
so I'm not willing to maintain it. I'll rely on Jenkins and, to a lesser extent, Travis.
Use motor_test DB in exhaust cursor tests.
Await replication when clearing a big test collection.

Debug some replication timeouts in later tests.
Skip Twisted resolver test in Tornado < 3.2.2.

Avoid a bug in Tornado's TwistedResolver implementation.
Mention backports.pbkdf2 as optional dependency.
More quickly determine if mongod is running SSL.
MOTOR-58 Demonstrate that pool can exceed max_pool_size.
MOTOR-58 Always enforce max_pool_size.