Show artwork for remote streams
Keep track of station icons to be displayed in Now Playing screens etc.
Aggressively cache artwork URLs for http streams.
Do a relatively cheap check on the cache before updating it.
Add support for service icons for services without protocolhandler

We need to register their icons together with a stream url regex to be
used in slimservice. Not a perfect fit, but helps in many cases.

Change-Id: I95c41543919b132cc51ab626860226d1232048ac
Keep track of station artwork when remote streams follow a redirection.
Change cache expiry - 'never' is unknown on mysb's memcached
Add RadioIO plugin to provide artwork for its channels.