$position push modifier support

[parkan]

Add support for $position modifier as described here http://docs.mongodb.org/manual/reference/operator/update/position/

We probably just need to something like

class ModifyPushEachPositionClause(fieldName: String, position: Int, values: Traversable[_])
    extends ModifyClause(ModOps.Push) {
  override def extend(q: BasicDBObjectBuilder): Unit = {
    q.push(fieldName).add("$each", QueryHelpers.list(values)).add("$position", position).pop
  }
}

class ModifyPushEachSliceClause(fieldName: String, slice: Int, position: Int, values: Traversable[_])
    extends ModifyClause(ModOps.Push) {
  override def extend(q: BasicDBObjectBuilder): Unit = {
    q.push(fieldName).add("$each", QueryHelpers.list(values)).add("$slice", slice).add("$position", position).pop
  }
}