Use Gulp instead of Grunt

Would prefer the unconventional gae-init way :)

```
clean: [
    "#{static_dir}/dst"
    "#{static_dir}/ext"
    "#{static_dir}/min"
  ]
watch: [
    "#{static_dir}/**/*.css"
    "#{static_dir}/**/*.js"
    "#{root_dir}/**/*.html"
    "#{root_dir}/**/*.py"
  ]
```

https://github.com/gae-init/gae-init/blob/master/main/config.py#L41-L43