reductions don't pollute scalar api
specify arithmetic operators to narrower class of dshapes
Merge branch 'master' of github.com:ContinuumIO/blaze
sql: use post_compute=False more diligently
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
Allow non reduction non collection expressions
Failing reduction on slices
Change reductions to use schema instead of _dtype
Only Reductions for now