feat(mock): use koa-mock for front end dev

demo: ![cnpm-pagemock](https://cloud.githubusercontent.com/assets/156269/5144334/808ddb62-71d4-11e4-9af7-506b17b96d18.gif)

Closes #509
Merge pull request #512 from cnpm/koa-mock

feat(mock): use koa-mock for front end dev