Add tested libraries as submodules

All submodules are checked out at correct tags:

- clogs 1.3.0
- VexCL 1.3.1
- Boost.Compute 0.2
- Bolt v1.1GA
- CUB 1.3.1
- ModernGPU 1.0

This allows to get everything necessary for benchmarking with
git clone --recursive https://github.com/bmerry/ssbench