expr: UTCFromTimeStamp
python: utcfromtimestamp
numpy: utcfromtimestamp
pandas: utcfromtimestamp
add utcfromtimestamp to comprehensive test
Merge branch 'master' into utcfromtimestamp
mongo: utcfromtimestamp

http://stackoverflow.com/questions/26678084/in-mongodb-how-do-i-convert-seconds-after-epoch-to-datetimes
Merge pull request #804 from mrocklin/utcfromtimestamp

Add utcfromtimestamp method/function
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