Max-ones argument and more detailed error reporting

I've added a couple minor features:

1. A check to make sure that the ones matrix is large enough to do a particular sum.
2. A keyword argument to set the size of the ones matrix during cublas_init if desired.
3. More detailed cublas error reporting (which mirrors the cublas errors in CUDA 6.0)
That sounds like the simplest and cleanest idea.  I'll try to get to that at some point with a separate PR.  Thanks for doing the thinking for me.  I didn't want to come up with anything too radical. :)