Unify the CPU, CUDA and OpenCL math functions API in the device wrapper classes

This PR wraps the math functions as suggested by both [Yangqing](https://github.com/BVLC/caffe/issues/382#issuecomment-42465152) in #382 and [jeffdonahue](https://github.com/BVLC/caffe/issues/408#issuecomment-43035934) in #408 to abstract the device type from the algorithms.
Oh, and I can work on this if it makes sense and we figure out an easy way to collaborate.