Adapt Key Manager's get_key() to the current nickserver spec.

• "get_keys pseudocode":https://leap.se/en/docs/design/nicknym#Pseudocode
• "example of client code using the webapp api":https://github.com/leapcode/leap_client/blob/develop/src/leap/services/soledad/soledadbootstrapper.py#L222
• "webapp api spec":https://we.riseup.net/leap/leap-provider-api#put-api_base-users-uid-json

(from redmine: created on 2013-05-20, closed on 2013-05-21)