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
Implement first/last reductions
Make first and last functions
xfail on last because negative slicing is not well supported in python
No need for SingleElementSlice anymore
Break out reductions to a single function
Implement slice for pytables Column
Add first and last to reduction test
Failing reduction on slices
Change reductions to use schema instead of _dtype
Add reduction usage test for first/last
Assume we have toolz latest
Make our reduction schema always be a datashape