Fix login/logout problem in spotify.c (see issue #68)
Remove misplaced quotes in db_group_type_bypersistentid()
Implement is_remote() and change how transcode_needed() is used
transcode_needed() was getting called needlessly in http_daapd.c,
because 1) once it is determined that a given codec needs transcoding
for a given client there is no reason to call and check again, 2)
transcoding is irrelevant for remotes. Also some cleaning up of
user_agent_filter().