Option to support webroot

Created by: mason-xx

first, I really appreciate your work on a new and working subsonic addon :)

Currently my subsonic instance is running behind a reverse nginx proxy with the following URL: https://host/subsonic.

The addon doesn't seem to take the webroot in account, all requests against the nginx go to /rest/... If I'll modify the libsonic/connection.py on line 100 and add the webroot hardcoded to the variable serverPath='/subsonic/rest' everything is working fine.

Since I'm not much of a programmer I don't know if you are able to work around that, or if this might be something for the maintainer of py-sonic?!