Let applications discover the user's PZH web address

This is a temporary measure to get Invitations working more smoothly
via requesting PZH information from the websocket.

Eventually, this will be replaced with a 'core API' to solve
security and privacy issues.

Jira issue: WP-855
Merge pull request #40 from jplyle/core-pzp-api

Let applications discover the user's PZH web address