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)
According to the documentation, `cublasInit()` returns a `cublasStatus` directly, you may want to use that directly.