document: Don't use mutable default values

http://effbot.org/zone/default-values.htm
document: Unused import
setup.py: Updates for MongoClient

MongoClient requires pymongo 2.4 or greater.
In addition, let's increment the minor revision level to 0.2.0 to signify that this version of the package will break clients who require/expect unsafe writes but don't specify safe=False.
docs.conf: Increment version from 0.1.8 to 0.2.0