Inline OpenIDServiceEndpoint.parseService
Return only one URI per service element when parsing XRDS.
Don't keep CanonicalID in an endpoint.

Per the spec, if it's present it must be treated as claimed_id.
Rename getOPOrUserServices → preferred_services, simplify implementation.
Get rid of the endpoint.display_id
Implement preferredNamespace using compatibilityMode, not vice versa.
Removed unused service method supportsType from the endpoint.
Move parsing and handling of canonicalID into fromServiceElement.
Don't store iname in the endpoint.
Extract common part of discoverURI/XRI into parse_xrds
Replaced xri.identifierScheme with xri.is_iname
Extract iname to url conversion into xri_url.
Inline preferred_services into parse_xrds.
Inline url building both for discoverURI and discoverXRI.
Accept arguments in OpenIDServiceEndpoint constructor, extract parsing pseudo-constructors as regular functions.
OpenIDServiceEndpoint rename:

- class renamed to Service
- methods and attributes renamed to more Pythonic names
More compact Service.__str__()
Move yadis.parse to xrds.get_elements as it has nothing to do with Yadis.
Removed Service.as_op_identifier.
Renamed test_yadis_discover → test_yadis, removed one useless test.