Tutorial Rewrite

This is my work on the rewrite of the tutorial, which I will incorporate in to
the SciPy SymPy tutorial that Ondřej and I will be giving.

This is still a work in progress, but I would appreciate any reviews even
before I finish, especially from native English speakers for grammar issues.

@certik, if you want to collaborate on this, just ping me. If anyone else
wants to collaborate too, just let me know.