Always set created on new documents

`created` is being given the same treatment as `modified`. Originally the `modified` field was updated by `save()` regardless of any preexisting value. `created`, on the other hand, only updated itself when no value was present (during an insert). Now `created` will automatically be set during an insert. Just as with `modified`, if this behavior isn't desired, `auto_timestamp` should be set to `False`.

Some related comments are being updated as well.

This is also my first attempt to updating the change log in conjunction with the change itself.
Simplify test_save()

`test_save()` is, as its name implies, a test of the `save()` method. Its goal is to make sure that `save()` calls `_update()` correctly. The pieces of the test that deal with `created` and `modified` are being removed so that the test can focus on what it needs to. `created` and `modified` are being covered in `test_save_timestamps()`.