connection: Replace Connection with MongoClient

Old and busted; meet new hotness http://emptysquare.net/blog/pymongos-new-default-safe-writes/
MongoClient is new and shiny. As far as functionality goes, the API is backwards compatible with the old Connection class so this change shouldn't break anyone's code. The major change is that writes are safe by default. If you want to use unsafe writes, set w=0. If you use the safe kwarg in your code, you'll get warnings since it has been deprecated in favor of the w option.
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
Merge pull request #10 from mdbecker/replace_connection_with_mongoclient

Replace Connection with MongoClient