Update tests for Meta class with proper write concern option

In order to fully deprecate the `safe` option when Simon is used with `MongoClient`, the `Meta` options class needs to set the `w` option instead of the `safe` option. When `safe` is defined, a `DeprecationWarning` should be triggered. These changes to the `TestMeta` should all pass once the necessary changes have been made to `Meta`.

At this point in time, tests should be failing.

References #17
Typo
Change Meta class to use write_concern instead of safe or w

This change handles storing a model's default write concern value as `write_concern`. It was originally handled as `safe`. The attribute is needed when calling `delete()`, `insert()`, or `update()`. Having to manage multiple attribute names would get messy. Taking care of it in `Meta` makes it much easier.

Part of this change is adjusting `ModelFactory()` to work with `write_concern` instead of `safe`. In addition, `ModelFactory()` is being updated to hopefully better handle things like what I saw in 68fc36d.

References #17