Improve the Complexity tutorial

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.)

Start with the Complexity tutorial at http://complexity.readthedocs.org/en/latest/tutorial.html and read it as someone who has never used the project. Note confusing explanations and flow issues, then review the relevant tutorial content. Done means the tutorial has clearer guidance and the improvements are submitted as a pull request, or specific feedback is provided in the issue.

Written by the indexing model from the issue text.