Add ContractList class

this class is inteded to hold all methods that currently are added to
the root Pacto module by the contract_repository file
Add ContractFactory as a dependencie to ContractList
Pacto root module now uses the ContractList class
Pacto#use and Pacto#register_contract are now chainable
Add Pacto#load
Busted by Rubocop!
Rename ContractList to ContractRegistry
Merge pull request #77 from thoughtworks/turn_contract_list_into_a_class

Turn contract registry into a class