initial reduceBy ideas

- reduceByRange, reduceByFile
- reduction of a list (like lapply), or iterative (X, Y)
- some parallel support
update reduceBy

- re-order MAP function arguments as RANGE, FILE, ...
- higher-level .reduce(..., BY=c("range", "file"))
- summarizeViews to return a SummarizedExperiment
  - implement mean() on BigWigFileViews
- re-factor .delegate
merge reduceBy branch into master