sympy.physics.quantum.pauli

This pull request contains a few minor fixes for the sympy.physics.quantum, and a new module for pauli operators, like sigma x, sigma y, sigma z, and the annihilation and creation operators sigma minus and sigma plus.

The pauli sigma operators are of course a special case of the angular momentum spin J operators, but I think they are deserve operator classes of their own because they are so commonly used. I have been looking at ways of making the new pauli operator classes subclasses of spin J classes, but I found that there was no advantage in doing that, because I had to reimplement all the methods and add new ones. However, earlier this summer when I discussed the  sympy.physics.quantum.fermion and sympy.physics.quantum.boson modules with @ellisonbg I also mentioned that I had worked a bit on these pauli operator classes and he recommended asking @flacjacket for advise and discussion on how these pauli operator classes could be related to the existing spin J operator classes.

So, @flacjacket, if you could have a look at this PR and give comments I would be very grateful. Also @ellisonbg  and @certik might be interested in having a look.

Here are a few ipython notebook that demonstrates a few applications of the new pauli operators implemented in this PR:

http://nbviewer.ipython.org/github/jrjohansson/sympy-quantum-notebooks/blob/master/lecture-sympy-quantum-two-level-system.ipynb

http://nbviewer.ipython.org/github/jrjohansson/sympy-quantum-notebooks/blob/master/lecture-sympy-quantum-atom-cavity.ipynb

http://nbviewer.ipython.org/github/eunjongkim/reproduced-papers/blob/master/Reproduce-PRA-80-033846-2009-Zueco.ipynb
Our general approach is to never add custom operator logic to the classes in quantum. The problem is that they can't be applied reliably in expression hierarchies. The best way to do simplicifations like this is to build a separate function that does that particular type of simplification. Also, this goes against the idea that no simplification should ever be done automatically unless every user will always want to have it done for them.