Update the tutorial

- Update the tutorial from my tutorial branch. Also the install and gotchas
  pages.

- I removed the finite difference stuff from master, because it isn't in the
  release.

- I ran runtests.py through 2to3. I didn't want to bother getting the latest
  SymPy version working, so this probably will only work in Python 3.
Update the index page with the latest information