More Aggregators for Ye Olde 0.7 branch

This is identical to the 0.7 release except that it has many Aggregators and Folds added.

See: https://github.com/twitter/algebird/blob/0.7.2/algebird-core/src/main/scala/com/twitter/algebird/Aggregator.scala#L55

There are now Aggregators for count, size, max, maxBy, min, minBy, forall, exists, sortedTake, sortedReverseTake, toSet, toList, uniqueCount and a few more.

Also, by an enrichment (for binary compatibility), you can .join two aggregators to aggregate a sequence (or a Scalding TypedPipe) without passing over it twice.

If you are not operating in parallel, the Fold type is just as useful. You can toFold and Aggregator, which can also be joined.

Aggregate away!