Add support for installing Alaveteli to install-site.sh

Along with changes in a separate pull request for Alaveteli,
these changes let you install Alaveteli using our standard
install script.

One change in behaviour is that the installation preserves
a copy of the script that when it's used, which means that
it can't be used when piped to `sudo sh -s`.  I've updated
all of our documentation to use separate invocations of
curl and the script, however.