Don't authenticate emergency registrations
Add utility method to check for emergency registration
Add allow-emergency-registration parameter, and have Bono reject emergency registrations when it's false
Only use star in contact header if expiry is 0
Add _emergency_registration to Bindings
Add emergency registrations as emergency bindings
Prevent user-initiated deregistration
Subscribe and Notify changes
Prevent registrations to application servers
Merge branch 'dev' of github.com:Metaswitch/sprout into sto755
 Updates after review
Merge pull request #533 from Metaswitch/sto755

[Reviewer: Alex] Sto755