Able to run buildout without breaking the running solr instance (#12)

This PR fixes #12.

As I said in the ticket, update() doesn't really do anything useful. It's handy if you want your recipe to run *every* time, but if you only want it to run when the part settings have changed then you only want the install() method.

This isn't enough because the uninstall()/install() is called every buildout (if you use it direct from git) or at least every time you change the schema or other part settings. So I don't tell buildout which files I created (to avoid its auto-uninstall code) and I now use a variant of copysolr that is safe to run over and over again, so that install is non-destructive.