v7.1.0: Interceptors and literal routes

• PR #276 implements interceptors, an elegant way to plugin new functionality to clients and services with a simple but powerful interface.
• PR #277, fixing #257 on the client side, allowing Go clients to talk to services implemented in different languages (e.g. Python) when the proto definition contains Service and Method names that are not CamelCased. Note that only Go services generated with v7 can handle literal routes, previous versions only support CamelCased routes.