Qubism examples notebook

Notebook explaining Qubism visualization and showing how to use it in QuTiP.

Tested in both Python 2.7 and Python 3.4.

See also https://github.com/qutip/qutip/pull/167 (without this PR this notebook still works, but a few plots with legend may look ugly).

I am open for comments and critique.

BTW: For my personal stuff, I've found function `spinchainize` to be clean and useful. I know I am becoming 'syntax sugar guy', but if you like it, I would be happy to add it to `qutip`.