install.sh and HTTP proxy for the plugins

[Cantor, Scott]

> It would also be nice to be able to do an offline plugin install or update
> from manually downloaded plugins.properties and plugin gzip files.

  --updateURL <URL>      Explicit location to look for update information (overrides the plugin's value)

Whether it allows file:/// I don't know, but it at least allows for alternate locations. And of course already allows for alternative HTTP client beans with proxying, etc.

The limitation is that the installer doesn't create a custom one for it when it's running the plugin checks.

-- Scott