This commit introduces 'accept_var_updates' that the VM_Linker uses to get
update expressions from the FunctionMaker class.  The previous code didn't
work right with ProfileMode, as triggered by the test_modes unit test.

Minor stuff

Updated NEWS.txt following scan change.

Fixed some tests

Temporary fix to a known bug for scan.

Added warning about sigmoid identification bug

Fix scan crash in recent change.

The code seems fine.,

Also updated NEWS.txt accordingly.

This fixes gh-302.

Test sigm opt

This fix the gpu scan optimization that was still passing an Apply as in the past.

Fixed bug in identification of sigmoid

It used to think that (any_constant + exp(x)) was equal to (1 + exp(x)).

Fix pycuda test

Fixed tests when on_shape_error = 'raise'

Scan check

It is expected that some implementations of 'infer_shape' raise a ShapeError to
indicate that they do not know how to compute the shape. In such a situation,
we should silently fall back to the default implementation, regardless of the
value of 'on_shape_error' (whose name may be misleading for developers, since
we want to catch all exceptions *except* ShapeError).

Also, when a bad shape was inferred, we should not raise a ShapeError (which
may be caught silently), but rather something that indicates that there is a
serious bug somewhere, like an AssertionError.

Minor typo fix and more consistent use of rand