Added support for WordPress networks and static defines

This modifies the get_option/set_option and get_transient/set_transient for config data (i.e. key, secret and token) with get_site_option/set_site_option and get_site_transient/set_site_transient. However, each site maintains it's own transient for the actual tweets in that site.

It also allows admins to define('RAPID_TWITTER_KEY') and define('RAPID_TWITTER_SECRET') in wp-config.php which will globally disable the configuration fields in WordPress Administration.