Review/tests for ExtractDiag and trace

added a hint to the BadOptimization exception

Fix div by zero in tensor grad

Correctly do version comparison: fix #136

misc cleanup and a bugfix in linalg ops

added sparse dot

Op doc merge

Conflicts:
	doc/cifarSC2011/extending_theano.txt
	doc/extending/op.txt

moved import cuda where I need it.

function

Op documentation

More explicit error message

Conflicts:
	theano/sparse/basic.py

Makes it more obvious to users what may be their mistake when they get a
TypeError after trying to index a shared variable.

Op doc

Due to lack of experience with git, I'll merge this pull request and send another one that fixes all the bugs I've seen.

itertools.product(), or theano.gof.python25.product() in this case.
Also PEP8 fixes.