namedirectory: use ns.ring.cx as default server

* drop support for hosting a server under a subdirectory
* allow uppercase username (set to lowercase in daemon)
* cleanup URI parsing, disallow space in URI

[guillaume.roguez: fallback ns.ring.cx for the IP, as the ns server is not ready yet]
Change-Id: I2fe86cba2faa79875f816ced108c1a0a4a7422be
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>