refresh requirements.  Add psutil
remove logging from __init__.py

cc @quasiben
Merge pull request #792 from mrocklin/remove-logging

remove logging from __init__.py
bcolz test specifies dtype explicitly
other bcolz test specifies dtype
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 {}