Headteacher Define Quota Feature Implemented
   - initialize method added to headteacher_define_quota_context
   - quota model test and quota model added
   - test_should_save_quota method added to headteacher_define_quota_context_test
   - Headteacher injected to person and HeadTeacher module added
   - call method added into headteacher_define_quota_context and define_quota method added into Headteacher module
   - exception handling implemented into call method of headteacher_define_quota_context