Improve the Complexity tutorial

[audreyfeldroy]

This is a first attempt at a Complexity tutorial:
http://complexity.readthedocs.org/en/latest/tutorial.html

It could use some reviewing from someone who's never used Complexity:

• What is confusing?
• What can be improved?
• What would make it flow better?

If you have feedback, comment here.

If you have enough experience to feel comfortable doing so, please submit a pull request with improvements. (Don't worry about this if feedback is easier, though.)