masterKeyWarn = function masterKeyWarn() {
  console.warn('MasterKey is not supposed to be used in browser.');
}
Call this method first to set up your authentication tokens for AV.
You can get your app keys from the LeanCloud dashboard on http://leancloud.cn .
@function AV.init
@param {Object} options
@param {String} options.appId application id
@param {String} options.appKey application key
@param {String} options.masterKey application master key