Add a plugin system #74 

This pull request allow to developp plugin for the admin panel of YunoHost. It is based on Sammy plugin system.
A plugin contains:
* a main.js file where the Sammy plugin is written
* a locale directory
* a view directory (but you can call base view if you need)

To activate a plugin you need to register it in modules/list.json.
This pull request contains a small HelloWorld you can call with https://yunohost.local/yunohost/admin/#/helloworld .