Use standard yadis service parsing instead of ProxyResolver().query() in XRI discovery.
Don't use xrires in discover.
Removed unused xrires module.
Inline xri_url.
De-couple obtaining of claimed_id from yadis.parse.

- claimed_id is obtained solely from yadis.fetch_data
- yadis.parse accepts raw XRDS data, becomes the only parse API
- endpoint generator becomes an internal helper function in
  yadis
Cleaned up the header part of discover.py
Removed unused http_response parameter from DiscoveryFailure.
Updated docstrings in yadis.
Removed version info from yadis.
Move DiscoveryFailure definition to consumer.discover.