expr: Summary
expr: Reductions support Number interface
compute: python arithmetic
api: start dplyr frontend
add dplyr source
add dplyr tests
python: summary tees of data sources

issue - this is non-streaming
dplyr: hook summarize to Summary object
python: compute summary tees only when necessary
dplyr: summary now returns by, not relabel.  Edit tests
dplyr: add reduce to imports
rename mutate -> transform
by: accepts keyword arguments to produce summary

    by(t, t.name, sum=t.amount.sum(), id_count=t.id.count())
add small docstrings to dplyr
import inject and transform from dplyr interface
Merge pull request #484 from mrocklin/dplyr

`dplyr` interface