Image cache and webroot support

This adds support for a couple of new optional features in TVH:

1. Webroot - allows TVH to server web pages from an alternative context, useful for those setting up TVH behind a rev proxy under a diff path.

2. Image cache - images (channel logos etc..) can now be cached by TVH. To keep things clean and consistent (since these images can also be accessed directly via HTSP) the URIs provided a relative to server root (without webroot). So HTSPv8 clients must prepend the protocol and server details.