(#2613, #2747) Add handling of negative axis for Join and GpuJoin

Maxpool c code

Better doc of function_dump

Fix print -> print()

Fix crash gpuadv inc1 broad y

Even on commented code. Just in case.
Also add __future__ imports.

Assert no CPU op for #2471

Change the elemwise input fusion limit to 32.

Fix nit spotted by @dwf

Fix warning reported on the mailing list

Negative axis were automatically handled by numpy implementation inside
Join.{perform,c_code}. However, both Join.grad and
GpuJoin.{perform,c_code,grad} implementations were not thought to take
care of negative axis.

This commit adds checks for exceeding negative axis in Join.make_node for
constants and in {Gpu}Join.{perform, c_code} for symbolic variables. Axis are
converted to positive values inside GpuJoin.{perform, c_code} since current
implementation does not support negative axis.

Respect PEP3113 (no more tuple unpacking arguments)

Replace except Class, e -> except Class as e