python: negative slicing with tail
install toolz/md from github master
python: slicing py34 compatibility fix
Add test for stop and step negative
Merge pull request #892 from cpcloud/mrocklin-python-negative-slicing

Continuation of #832
update dependencies
explain dependencies
ENH/BUG: add context manager for expressions to auto-close resources
style changes for contextmanager test
replace dict.values indexing with toolz.first