document: Remove safe kwarg

All inserts default to safe so this logic is no longer necessary.
If you were previously setting this field as an arg instead of a kwarg, this change will break your code.
Additionally, the safe kwarg is deprecated, so you should switch to using the w flag instead.
document: Unused import
document: Don't use mutable default values

http://effbot.org/zone/default-values.htm
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