document: Replace safe_insert

The safe_insert attribute is deprecated in favor of the new write_concern_override attribute.
Unlike safe_insert, write_concern_override will impact all writes, not just calls to the save method.

I changed cls to mcs in the metaclass because mcs is the standard (see http://goo.gl/Cvo69)
document: Remove safe kwarg

The safe kwarg is deprecated, so you should switch to using the w flag instead.
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