hack inspect, display all the apis

use [show-methods](https://github.com/dead-horse/show-methods) to list all the methods, accesses, getters and setters of ctx, ctx.request, ctx.response. 

related to #245, it is a little weird to list in `console.log`, but seems no easy way to do this now.
 a koa executable is awesome, but I think most people probably won't use it? 
and the `console.log` can also list all the methods that other middlewares add to `ctx`