Clean up the docs a bit

Mostly typos, here's the real changes:
* For travis-ci, test with `iminuit/iminuit`, not `piti118/iminuit`
* Make the example in the README work and add the same example to the top-level docs page
* Add license info to README

Concerning the naming changes in the basic example ... I'll make a PR for an updated tutorial IPython notebook shortly where we can discuss this.