[Ur] time measurement

[Adam Chlipala]

Gergely Buday wrote:
> As I need it now for measuring page regeneration on the client side, I
> have made the following patch:
>    

Thanks!  I've added a modified version of your patch.  (IMO, the right 
function name is [diffInMilliseconds], since "milliseconds" is a single 
word.)

Two quick requests for future patches:
1. Generate them with 'hg export', so that they are trivial to import 
into Mercurial repos while maintaining authorship information.
2. Include patches as attachments, not inline in message bodies.