Fix typos in docs refs pr #65
Support Aggregation for MongoDB 2.6

* Added support for Aggregation returning a cursor `AggregationCursor` (SCALA-117)
* Added support for `allowDiskUsage: true` to the top-level of an aggregate
  command with the AggregationOptions helper (SCALA-128)
* Added support `$out` aggregation pipeline operator (SCALA-130)