ensure expire keys are removed on flush. closes #13.

This seems to work...

I just copied what you had in the [`cleanup`](https://github.com/d0ugal/locache/blob/master/locache.js#L636) method.

Also for the tests, I mostly copied your [cleanup expired values test](https://github.com/d0ugal/locache/blob/master/tests/specs/expire.js#L79).

Let me know if you see anything wonky.