[Rivet] HepMC, cmake and our bootstrap script

[David Grellscheid]

> Phew. I was thinking we'd have to see if we could set/fake the
> User-Agent HTTP header with urllib2...

Yes, just what Frank did and it still didn't work. I think it's fine to 
keep the python user agent string, but we could add 'Rivet x.x.x' at the 
end of the annotations. (LHAPDF-getdata needs the same!)

   David