Add support for $comment in find().

This adds support for $comment:
http://www.mongodb.org/display/DOCS/Advanced+Queries#AdvancedQueries-%24comment

$comment is a great aide in debugging slow queries.

To test, i verified the existing tests passed and ensured via profiling that the comment field was being properly passed along/logged.

To enable profiling for all queries (not just the slow ones):
```
$ mongo
> db.setProfilingLevel(2)
```

To view the logged queries:
```
$ mongo
> db.system.profile.find()
```