[Ur] Ur/Web on GitHub

[Adam Chlipala]

I've finally started moving the Ur/Web project over to GitHub.  The plan 
is to substitute standard GitHub procedures for all current uses of 
Mercurial, Mantis, and the impredicative.com wiki.

As a first step, the main repository is imported into GitHub here:
     https://github.com/urweb/urweb
I hope that all future changes will be made in Git, with that repository 
as the canonical source!

Does anyone see any issues that stand in the way of immediately 
switching version control and issue tracking to GitHub?  ("Moving" the 
wiki would be a less automatic process.  Probably not all of the current 
content should be moved, which is actually also true for the current 
Mantis tracker, IMO.)

I'll wait 24 hours from sending this message for any words of caution.  
If none arrive, then I'll switch Ur/Web over to GitHub officially, 
updating the project home page.