Fixed the compiling bug for CPU without CUDA GPU.

Added #ifdef HAS_CUDA for gpu codes.
Disable building apps for temporarily.
Added SigmoidForward CPU native impl.
Added ReluForward CPU native impl.
Added TanhForward CPU native impl.
Modified minst_mlp when CPU only.
Enable compiling mnist_mlp when CPU only.
Fixed coding style issue.
Split the unittest to Gpu and Cpu.
[xianyi-pure_cpu] Merge branch 'pure_cpu' of https://github.com/xianyi/minerva into xianyi-pure_cpu
[xianyi-pure_cpu] Merge
[xianyi-pure_cpu] YCM file
[master] Merge branch 'xianyi-pure_cpu'