Clean up service code; authService handles default headers

The original goal was to make the status bar show the right thing
(username/logout) after sign up. Turned out there was an error
occuring while trying to record the session token that identifies
the user in subsequent parse.com requests. I ended up refactoring
to remove some duplication and make authService responsible for
setting up the default config used for all services.

A side task was to make the sign up process set the user name so
the right one shows up.