Added Labrador model and associated methods/views

Added the basic Labrador model which will be expanded later. Added CRUD
views/methods for easy updating by admin as well as showGirls/Boys
which will be publicly viewable.
Added Litter and Litter/Labrador Association

Added basic litters to the application. Set up an association with
Labradors (as mother and father of the litters). Have basic CRUD
functions as well as methods/views for displaying the current and
future litters. ‘Current’ litters are any litters that have been born
but are not yet 12 weeks since the puppies will be going home with
their puppy parents between 8 and 12 weeks.