add test to ensure that server supports datetimes
Merge pull request #796 from mrocklin/test-server-supports-datetimes

add test to ensure that server supports datetimes
Add length expression
Add NRows expression
Implement NRows for different backends
Clean up star imports
Move to reductions
Rename NRows -> nrows
Use None instead of {}
Import cleanup
Move to reductions
Axis is already there
Make nrows a property on tabular things
Use nelements() and nrows property
Remove the NRows class
Add better docstring
Add nrows with nunique
Implement SQL nelements