Rename Request to RequestClause

For a better separation in the codebase between an actual request,
like Faraday's request and a contract request part
Rename Response to ResponseClause

To differentiate between the contract response part and actual http
responses
Rename Pacto.build_contracts to load_contracts
Rename BuiltIn to be WebMockAdapter
Merge pull request #98 from thoughtworks/naming

Some Naming Changes