Overhauled documentation

This is a complete overhaul of the documentation. It adds tutorials generated from the notebooks in trackpy-examples, various information from the README, much-improved API docs, sample images from various groups' work with trackpy, and a modern, bootstrap theme.
"here" seems like a very small click target for such an important thing. How about

[**Read the documentation**](http://soft-matter.github.io/trackpy/) for an introduction to the project, easy installation instructions, tutorials, the reference guide, and much more.

(OK, I also took out some of the items you listed. Feel free to add some back in.)