added $first, $last, $before and $after query parameters

I need to browse through the database efficiently and using sort, skip and limit is inefficient as they make a lot of copies of documents internally while making the query.

These query parameters are slightly different than the rest in that they always only give you one result, but allow for very efficient browsing through the index. 

These can also be used to kind of "join" two NeDB databases manually by querying them in parallel and finding matching / missing documents.

Feel free to merge if you will; please note that there is a corresponding pull request to node-binary-search-tree which is required to be able to do this.