add --base-url option to serve command, thus making it possible to place plovr behind a reverse proxy

This updates CompilationServer's getServerForExchange() so that it can
return a fixed base URL instead of re-constructing it from the referrer
header in the request. In so doing, we make it possible to serve plovr
from behind a reverse proxy. For instance, if your development server's
nginx is configured with:

	location /static/debug/plovr/ {
		proxy_pass http://localhost:9810/;
    }

you may now configure plovr such that the URLs it generates are prefixed
with http://localhost/static/debug/plovr/ by running:

    java -jar plovr.jar serve \
        --base-url https://local.dev.foo.com/static/debug/plovr/ \
        plovr_config.json