change Connection behavior into promise based to resolve async auth issue

[ianwang]

Problem

If the client makes db connection before auth success, the current implementation will not be able to subscribe successfully, and failed without any error messages.
The problem is that the auth behavior is async, so need a way to let client connect to realtime database after it passes auth checks

Major change

Before

const channel = subscribe('/my-test-path');
// at this point, client might be still checking auth data
channel.on('child_added', function(val) { 
  console.log('on child added', val);
})

After

subscribe('/my-test-path').then((channel) => {
  // here we can make sure user has already logged in!
  channel.on('child_added', function(val) {
    console.log('on child added', val);
  })
})

Patches

• use standard to lint file
• use babel to transpile for output instead of webpack
• use babel-preset-env instead of babel-preset-es2015