Remove first argument to `by`

I've tried to make all the changes to remove the first argument to `by` from discussion in #517 

Lots of small changes in many files, so, it's likely that I forgot something.
>This PR should not need to touch any of the compute implementations.

How about here in compute/chunks.py, I've changed those two `by`
https://github.com/ContinuumIO/blaze/blob/master/blaze/compute/chunks.py#L178-L203