make TypedPipe, KeyedList and Grouped covariant in T
add TODOs for aggregator type variance
add comment re invarance of scala Set type parameter
fix type parameter of operate in Grouped.reduce
Fixes buggy fromMillisecs
actually fixes
adds some methods, improve the tests
Merge pull request #265 from asiegel34/typed-pipe-type-variance

make TypedPipe, KeyedList and Grouped covariant in T
Merge pull request #269 from johnynek/bugfix/fromMillisecs

Fixes buggy fromMillisecs