Implement (bilinear) upsampling

Hi! For all the DL libraries out there, as well as the image processing packages it would make sense to have GPU-powered upsampling (nearest, bilinear, cubic), including their derivatives. Being able to do this in any number of dimensions would be nice of course, but I think 2D would do it to get going finally. From the Flux side the issue is tackled in [this](https://github.com/FluxML/Flux.jl/pull/1180) PR. I ported the Caffee 2 kernels for bilinear upsampling from pytorch in [this](https://gist.github.com/maxfreu/815c8e0c8304bab39d7f78033289cece) gist. Please have a look and decide if you want to have it here in some form and tell me which changes to make and where to put it eventually. I could then set up a PR, including more rigorous tests. Note that there is an older attempt [here](https://github.com/JuliaGPU/CuArrays.jl/pull/293).