This only remove warning from DebugMode.

fix first part of gh-842

initialize the destroy map to empty

No NEWS.txt. This is related to change in the trunk and happen only when we try to use Theano without g++.

Support for undefined gradients

Mixed3

(needed to express that gradient with respect to x is undefined
if x is allowed to be None)

added a new kind of bad grad, undefined grad

We don't want to do it at import time.

renamed FunctionGraph.extend to FunctionGraph.attach_feature

made extend call attach_feature

Tell how to get the full list.

No need to review in detail. This is sandbox stuff.