Convolutions and Pooling

See [here](https://github.com/JuliaImages/ImageFiltering.jl/issues/52) for a description of the operations involved.

I expect that pooling will be relatively easy, convolutions are also straightforward enough in principle but hard to make really fast. We probably want to port or wrap an existing high-quality implementation for Open CL, but I'm not sure what the benchmarks is here (perhaps [DeepCL](https://github.com/hughperkins/DeepCL)?).