Enable write concern by default

PyMongo 2.4 enabled write concern by default. Simon had already implemented the previous default behavior as its default as well. As part of the larger effort to better utilize `MongoClient`, the behavior is being switched to use write concern by default. This will also apply to earlier versions of PyMongo that only offer `Connection`.

References #4
Update tests for new write concern behavior

Most of these tests were written at a time when the database was required, so write concern was explicitly enabled. Because the database isn't necessary anymore, the default behavior can be used.

Plus new tests are being added to make sure that write concern is respected by methods that use it.

References #4
Remove unused import

The `Collection` class is being removed because it is no longer needed as a result of 5abf612. It could have also been removed as a part of c29ecbd.