$where operator

This adds an implementation of [MongoDB's `$where` operator](http://docs.mongodb.org/manual/reference/operator/query/where/).

Major differences between this an MongoDB's `$where`:
* Using `obj` instead of `this` isn't supported.
* Using strings instead of functions instead of functions isn't supported (to avoid using `eval`);

I tried to maintain the same style as the rest of the code, but please let me know if there's anything that requires cleanup.
Docs also updated.