Use Gulp instead of Grunt

```coffeescript
root_dir = './main'
static_dir = "#{root}/static"
paths = 
  clean: [
    "#{static_dir}/dst"
    "#{static_dir}/ext"
    "#{static_dir}/min"
  ]
  watch: [
    "#{static_dir}/**/*.css"
    "#{static_dir}/**/*.js"
    "#{root_dir}/**/*.html"
    "#{root_dir}/**/*.py"
  ]
```