birth cohort state processing #3453
use #ineligible in views #3459
use #ineligible, update state by #advance #3453
remove obsolete #update_state_after_survey #3453
specs for #3453, #3459
clean-up based on feedback #3453

Fixed typos, removed inappropriate use of 'let', 
made #pbs? private
renamed #eligible? call to #ineligibile? 3453
Merge pull request #56 from NUBIC/change_3453_change_participant#advance_to_handle_birth_cohort_transitions

Use #advance for state transition, eliminate of #update_state_after_survey, #3453