handle more connection parameters

Hi Dan,

Would you please consider those commits ? They add connection parameters support for :
- all newer pymongo ReadPreference flags as per [1]
- use_greenlets flag for the gevent users
- socket and connect timeouts (ms)

[1] http://api.mongodb.org/python/current/api/pymongo/index.html#pymongo.read_preferences.ReadPreference

Thank you :)