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)
Yeah, I agree.  I'll remove those two extra errors and just describe the specifics in the exception message.