This will enable CPU-only Caffe compilation (#3), and go a long way to 10.9 CUDA compilation problems (#122)

Checklist [shelhamer]:

• split layers into cpp and cu Splitting source files between CUDA and CPU code. #172
• split enough for boost-eigen to build on osx (hide boost rng from nvcc) MKL/non-MKL Reconciliation #165
• split and tag tests as CPU / GPU
• split math_functions into cpp and cu
• split common into normal and cpu-only singletons?

Follow-up: abstract CPU / GPU device computation #610