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. I've also replaced the existing ``safe_insert`` attribute with a new ``write_concern_override`` attribute. Unlike ``safe_insert``, ``write_concern_override`` will impact all writes, not just calls to the save method. Usage of ``safe_insert`` will print a ``DeprecationWarning``.