configure Makefile for CPU-only build
stub out GPU layer methods to crash loudly in CPU-only mode
add guards to drop GPU code in CPU-only mode
collect CUDA includes and calls, separate from CPU-only mode, leave out

- collect CUDA includes in device_alternate.hpp
- add guards for CUDA code
- move GPU code into cu from cpp
- make CUDA includes and libraries conditional in Makefile
- drop CUDA dependency from travis-ci build
switch travis build to CPU-only flag, drop runtestnogpu target
relax benchmark test timing for cheap hardware / CI build
check CPU-only everything, CPU + GPU build with travis
Merge pull request #561 from shelhamer/cpu-only-build

CPU-only build

no GPU, no CUDA, no problem!