add callbacks on memberships for hipchat

[#68095508]
configuration for hipchat

[#68095508]
add msg for membership created

[#68095508]
add msg for membership removed

[#68095508]
HipChat::Notifier#send_notifications method

[#68095508]
send notifications on create/destroy callbacks

[#68095508]
fake hipchat token in conf so tests will pass

[#68095508]