Pull request

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)
The advantage of the old implementation was that the exception was always raised in the function that caused it. Your implementation makes the code somewhat shorter, though. I'm fine with either way.