refactor method 'is_eligible_for_renew'
refactor method max_renewal_length_available
remove unused functions in reservations controller and functions that could be replaced by scopes
move user methods to user model
slightly refactor status
fix line error
put cartreservation back..
forgot an = sign
Merge pull request #614 from YaleSTC/614_refactor_res

Refactor Reservation Model.