BUG: Allow renaming of a column after mapping

Related #511

This should be merged before deciding on a less verbose way to rename things.
Some discussion on spellings for `map` here https://github.com/ContinuumIO/blaze/issues/513

I don't understand the data descriptor comment.

One issue is the combination of 

1.  We can't infer anything about arbitrary Python functions 
2.  We don't handle type ignorance well

Not too much we can do about 1.  Maybe something we can do about 2?  My solution is just to make spelling the schema more natural somehow.