Add array and uarray as aggregators

These are both trivial with --mr-agg snippets but require much more
typing than -a uarray.

Based on concat and uconcat, but without the final join.  There's a lot
of refactoring that could happen here using roles for class composition.
Test Array and UniqArray aggregators
Merge pull request #44 from benbernard/tsibley/array-aggregator

Provide two new aggregators: array and uarray