Fix gh-2210, Join.grad crash with first input dtype being *int*

Documentation : error messages

Update extending_theano tutorial

[BUG] Fix bug introduced yesterday, fix FFT conv optimization and small pydotprint fix.

Adding new superclass for C ops

subgrad documentation moved from tensor to theano.gradient

This relax a condition in DimShuffle to help reuse it.

We where removing the DimShuffle, it was working when it was only
adding outer dimensions, as the Elemwise would readd it. But if it did
other stuff, we where loosing what it did.

PEP8/flake8 fixes in tensor/basic.py

Relax comparison of pickled size.