Initial Swagger support

Adds initial support for Swagger. This just maps Swagger concepts to existing Pacto Contract concepts. It doesn't take advantage of the swagger concepts that dont' have existing Pacto equivalents.

I'd like to deprecate the current contract format and replace with Swagger 2 as the default format, because Swagger is able to express a lot more information than Pacto - and supporting Swagger opens Pacto up to a larger community and interoperability with others tools.

There is also support for the Pacto server. If you have swagger docs in swagger/, you can run:
`bundle exec pacto-server -sv --format swagger --stub --contracts_dir samples/swagger/`