[Ur] Beginning of Ur tutorial

[Adam Chlipala]

I've put up the first two chapters of an Ur/Web tutorial:
     http://www.impredicative.com/ur/tutorial/
In particular, what's up there now should be enough to explain all the 
core features of the language (to people with good ML and Haskell 
familiarity), including the mind-bending statically-typed 
metaprogramming.  Please let me know if it makes sense!

I also want to point out that this tutorial HTML is generated from 
literate Ur source by a new compiler flag supported in the working 
Mercurial repo.  You're welcome to use the same functionality to help 
create your own tutorials!