Simplify ContractRegistry - remove tag support

There's no longer any documented way to use tags.  Since adding support for ContractLists in https://github.com/thoughtworks/pacto/pull/77, we've just been using lists instead.  This kills off all the dead code, basically turning ContractRegistry into a Set of Contracts with one helper method.

This makes searching a lot easier, which makes it easier to write rspec matchers.  I don't see a need for tags at this time, but they could be easily reintroduced by storing the tags on the contracts:
```ruby
def find_by_tag(tag)
  Pacto.contract_registry.select{|c| c.tags.include? tag}
end
```