Use native cuda atomicAdd on Pascal (#174)

Fixes #125

* Add `cuda_atomic_add` and `cuda_atomic_sub` wrappers for atomic addition.
* Choose native atomic add for Pascal and later architectures.
* Choose CAS workaround for devices earlier than Pascal.
* Add unit test for wrappers.
* Change default CUDA architecture target to `sm_60` in `CMakeLists.txt`.