Aggregations

[anovstrup]

This pull request adds various aggregation operations. There are variants that produce cumulative results (scan, scanAggregate, and scanAggregateBy) and ones that operate over bounded windows (fold, aggregate, and aggregateBy). Also included in the pull request are basic syntax/ops for tuples and for strings, which will be necessary to implement a complete WordCount example. The essence of the word count example is implemented in the pull request as a handful of defs in the SimpleExample module, but a complete implementation in WordCount.scala is not included.