Add autoconfiguration for gRPC servlet

[dsyer]

In a web application, if there are BindableServices we use
a servlet instead of a netty-based server.

Fixes #18

[onobc]

This is cool stuff @dsyer . I have added a few comments/concerns.

[onobc]

There are limited options for the ServletServerBuilder so providing a custom server factory is overkill since the ServletAdapter#init does the same thing we would be doing:

  public GrpcServlet(List<? extends BindableService> bindableServices) {
    this(loadServices(bindableServices));
  }

  private static ServletAdapter loadServices(List<? extends BindableService> bindableServices) {
    ServletServerBuilder serverBuilder = new ServletServerBuilder();
    bindableServices.forEach(serverBuilder::addService);
    return serverBuilder.buildServletAdapter();
  }