Support Pingdom's multi-user auth system.

https://www.pingdom.com/features/api/documentation/#multi-user+authentication

Multi-user auth is used if the 'username' function to API calls is of
the form:

    subacccount@example.org/masteraccount@example.org

ie with a literal / separating the sub from master accounts.
Merge pull request #7 from atlassian/multi-user-auth

Support Pingdom's multi-user auth system.