Implement $push 2.6 support with $each, $position, $sort, and $slice.

This brings $push support up to the 2.6 level, slightly tweaks the $sort order per the [2.6 docs](http://docs.mongodb.org/manual/reference/operator/aggregation/sort/), and adds test coverage. ```mvn test``` passes.

Changes:
* Implements Mongo 2.4 introduced $push with [$each](http://docs.mongodb.org/v2.4/reference/operator/update/push/), and 2.6 introduced [$position, $sort, and $slice](http://docs.mongodb.org/manual/reference/operator/update/push/).
* At some point DBList increased in sort comparison order over DBObject: [link](http://docs.mongodb.org/manual/reference/operator/aggregation/sort/).