made extend call attach_feature

fixed feature class versus instance issue

Adds correct handling of unimplemented gradients

of instances thereof

disallow uncomputable ops)

added an optimization to stabilize log softmax

optimization

Mixed2

Conflicts:
	theano/gradient.py

Using a Feature to catch BadGradOp

Better doc of extra_ops.

added error checking and doc to reshape

documented the images2neibs ops

Change replace_all_validate to replace_all_validate_remove

Minor stuff