CMake CPU-Only build

- `CPU-ONLY` CMake build option to facilitate #561
- CPU-Only build added to Travis-CI
- Tests of the CMake CPU-Only build under Travis-CI

In order to build CPU-Only version of Caffe you have to run:
`cmake -DCPU-ONLY=ON path/to/CMakeLists.txt`

When `CPU-ONLY` is enabled no CUDA libraries are looked for or linked to any target, nor are any `*.cu` files compiled.