don't accept the DEFAULT value for axis and update the docstring for theano.{max,min,argmax,argmin,max_and_argmax}

Fixed optimization for exp(x) * sigmoid(-x)

Merge branch 'documentation_equal_computation' of https://github.com/pascanur/Theano into pascanur-documentation_equal_computation

Conflicts:
	theano/scan_module/scan_utils.py

rebased Razvan's Rop/Lop pull request
It was checked by me and Ian.

New user-friendly function to compare variables

Great commit. Everything looks nice. We should make a ticket though that at one point in time we should remove one of the two implementations .. 

Fix test

Minor

Unlikely to make any timing difference, but makes it slightly easier to read
the code.

Since the 'finally' block requires 'add_stack_trace_on_call' to be defined,
technically it is better to define it out of the 'try' block. Most likely
doesn't matter here though.

fix bug in equal_computation when comparing a variable with itself

Fixed test_linalg.py:test_spectral_radius_bound

This stability optimization didn't work for at least three reasons:
1. It used tensor.neg(x) to identify -x, while in canonicalized form -x
   is written -1 * x.
2. There was a typo (calling a function sigm that didn't exist).
3. It wasn't able to parse a tree of multiplications, for instance
   something like (exp(x) * 2) * sigmoid(-x).

This new version fixes all three issues. In order to solve problem #3,
it uses a different approach that consists in parsing the optimized node
into a so-called "multiplication tree".

Currently, the structure of this tree is left unchanged. It would be
possible to have this optimization also flatten the tree, which would
make the code simpler (but potentially undo work performed by some other
optimizations).