Provides an instance of this class.
The instance of the class created with this constructor will send the events on the fly.
Also, if a request returns an error, this will be logged but the text will be lost.
token is the token associated with your project.
onError is a callback function that will be executed in case there is an error, otherwise debugPrint will be used.
prefsKey key to use in the SharedPreferences. If you leave it empty a default name will be used.