New assertions: supersetOf & subsetOf

##### New BDD assertions:

supersetOf

    ['pepperoni', 'tomatoes', 'onion'].should.be.a.supersetOf(['tomatoes'])
   
> (*validation of pizza toppings*)

subsetOf

    ['air-conditioning', 'wifi'].should.be.a.subsetOf(['gym', 'wifi', 'air-conditioning', 'hydro'])
  
> (*validation of requirements for an apartment search*)