Reorganization + Firebase schema + Arduino communication

+ Splitted project in different files
+ added a `package.json` file
+ loads schema from firebase, Ex: `{ status: { contentType: 'U8', fieldName: 'status', messageType: '0x50' },
  weight: { contentType: 'U8', fieldName: 'weight', messageType: '0x50' } }`
+ receive data from Arduino and update Firebase
+ send data to Arduino when value changes on Firebase