improved mercurial formula

I was trying to use fast-export (see e.g. http://stackoverflow.com/questions/10710250/converting-mercurial-folder-to-a-git-repository) with a homebrew-installed Mercurial, but it didn't work, because it couldn't find the "mercurial" module. I changed the formula to work more like a normal Mercurial installation, placing the mercurial module in site-packages.

I use the system Python; it would be nice if someone using a homebrew-built Python could test this too.