Add a gridsize() CUDA helper function to compute blockDim * gridDim for loop strides.

Analogous to the grid() function used to compute global thread offsets.
Add support for atomic addition on float32 values in CUDA Python.
Fix atomic tests to use correct device functions
Add documentation for cuda.atomic.add.
Merge branch 'gridsize' of https://github.com/seibert/numba (#444)
Merge branch 'atomic_float' of https://github.com/seibert/numba (#459)