Created Booking model

Each instance represents a single booking for one lawn at a particular time
on a paricular day. Each booking has a flag denoting whether or not the
booking has been booked by a user.
Bookings model under control of active admin gem
Model must have a description field set
All fields of each booking must be set
Each lawn has many bookings associated with it
Each booking belongs to one lawn instance
Added to string method to Lawn model

Simply returns the descriptin field
Don't validate presence of booked field. Instead use defaults
Generate and run migration setting bookings booked default to false
Created two sample bookings for each lawn