Creating answer as part of registration

Using registration entity answer set to create answer
Removing registrations onetomany relationship from ConferenceEntity

This relationship is better reflected as manytoone from registration
entity
Show Registration mantoone relationship to Conference

Changing registration to conference relationship from onetomany to
manytoone, since that better reflects intended usage (we don't really
want all the registrations everytime we lookup a conference)
Adding registration creation to conference resource
Moving answer creation test to registrationservicetest
Change RegistrationServiceTests setup to before/after each method
(since there are unintended side effects when sharing same persistence
context across methods).