CuDNN: Support for asymmetric padding?

**Problem**
Currently, asymmetric padding doesn't work in Flux when using CuArrays:
```
using Flux, CuArrays
m = Conv((3,3), 1=>1, pad=(0,1,0,0)) |> gpu
x = rand(100,100,1,1) |> gpu
m(x)
```
throws an error (when run on a gpu): `CUDNNError(code 3, CUDNN_STATUS_BAD_PARAM)` and gives the warning `Warning: CuDNN does not support asymmetric padding; defaulting to symmetric choice`. This means that

1. my current model (which runs on the CPU) cannot be trained using a GPU (without modifying the code)
2. there is no longer a one-to-one correspondence between CPU valid code and GPU valid code. (I.e., the above code will run just fine on the CPU if there is no GPU available.)

**Desired solution**
Well, CuDNN support for asymmetric padding would be great. Then I wouldn't have to worry (as much) about whether code developed on a CPU will work on a GPU.

**Alternative solution I've considered**
One can obviously use symmetric padding and then crop the output to get the desired result. This is however not very elegant, nor as efficient (?).