Remove unused import

The `warnings` module was added to the `test_meta` module as part of 65dbad1.
It had been added in preparation for `Meta` triggering `DeprecationWarning`. It
will be needed later to properly test for the warning, but it's being removed
for now because it isn't used by anything yet.
Deprecate safe meta option

As part of changing from `safe` to `w` with `MongoClient`, the `Meta` class
will now fire a `DeprecationWarning` when `safe` is included in a `Model`
class's `Meta` class.

Versions of PyMongo prior to `MongoClient` (2.3 or older) will not see the
`DeprecationWarning`.

References #17
Pay not attention to the man behind the curtain

This test makes sure that the `Meta` options class triggers a
`DeprecationWarning` for versions of PyMongo with `MongoClient`. Unfortunately
the warning seems to be triggered by another test first, and so
`warnings.catch_warnings()` is unable to catch the warning during
`TestMeta.test_safe_deprecationwarning()`. As a (hopefully temporary)
workaround to this, Mock is being used to mock the warnings module to ensure
the correct warning is being triggered.

References #17