Replace APP_CONFIG[:terse_pod_url] with uri object.

Adds a new APP_CONFIG[:pod_uri] item, an uri object parsed from
pod_url. Replace all occurrences of APP_CONFIG[:terse_pod_url] with
APP_CONFIG[:pod_uri].host. Closes http://bugs.joindiaspora.com/issues/684,
using the well-defined semantics of the uri object.

The pod_url is normalized using module URI's functions, always with a
trailing /.

The diaspora-handle will always reflect the pod_url with this patch
i. e., a pod_url like www.dpod.se will give the handle xx@www.dpod.se;
previous code stripped the www. prefix. If this is a problem, it
should be addressed by another setting, since one cannot presume that
www.domain.tld resolves to the same address as domain.tld.