Fix other crash in hessian of stack()

mod derivative

Fixes in downsample.py for python 3

Few changes to mod derivative

Add c code to rebroadcast.

This also fix a memory leak when Theano gc is disabled.

Convert string literals for Python3

Add missing include in the new gpuaelemwise op.

It was causing compilation crash in the DLT mlp

This one worked on Linux, but not on Windows, for some reason.

In RepeatOp, fix detection of numpy-supported dtypes

GpuShape

c_code() for GpuCAReduce (in gpuarray)

Add tests for DownsampleFactorMaxGradGrad

Added second derivative to max downsampling function

- call verify_grad on DownsampleFactorMaxGrad
- call hessian on DownsampleFactorMax

This is not pretty code, but it is working code.