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
Keep symbol
Remove first and last functions
Merge pull request #886 from cpcloud/first-last-reduction

Implement first/last reductions