steam: removed 'server_url' setting

This option sounded like a decent idea at the time for fine tuning the
plugin. However, in reality it is rather impractical, and has been
removed as a result.
steam-api: ignore empty lists for friends and summaries

Currently, errors were being thrown for an empty list of friends or
summaries. This is problematic as the Steam Mobile API occasionally does
not return an initial friends or summaries list. Instead, it will notify
the the plugin of friends and their statues via the subsequent poll
requests.