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
Test for prettier datetime truncate repr
Implement prettier datetime truncate repr
Validate truncate arguments in a more informative way
PEP8-ify
Merge pull request #897 from cpcloud/datetime-truncate-repr

Prettier datetime truncate repr
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