Fixes #586, the error was not due to the values being compared, but to the
comparison function itself.

See http://projects.scipy.org/numpy/ticket/1684 if you're curious.

tests.

tensor/test_basic:T_scalarfromtensor.test0 which was already a known failure

This should unbreak GPU shared variables, broken after [c4a975e14f7c].

None is almost like False, except it is OK do downcast a Python float to
a scalar floatX array. Other specialized case might be added later.

Change all theanorc code blocks in docs to use the 'cfg' Pygments filter which is designed for this.