Changes

tsk63 Invitations between team and competition
Eliminates the many-to-many relationship previously created. This 
happens because there is an attribute between the relationship, so it is
have to manage this manual relationship to work with composite keys.
Adds that a competition can add or remove an invitation to a team, and a
team can accept ot reject that invitation.
Adds the new entity to the persistence.xml and test-persistence.xml 
files.