Hi, 

Currently in the Java Sender Library the pushApplicationId and the masterSecret are part of the UnifiedPushMessage object. I would like to move these 2 fields to the Sender object and add it to its Builder API, to have something like :

JavaSender defaultJavaSender = new SenderClient.Builder("http://localhost:8080/ag-push")
                .pushApplicationId("c7fc6525-5506-4ca9-9cf1-55cc261ddb9c")
                .masterSecret("8b2f43a9-23c8-44fe-bee9-d6b0af9e316b")
                .build(); 

Why ?

• Regarding design, these 2 fields are more part of the sender rather than the message itself
• This is a first step to externalize the configuration, so we could have later, for instance, `JavaSender defaultJavaSender = new SenderClient.Config("path.to.config(or resource stream")

What does it change ?

• It only concerns the Java Sender, we do not touch the Rest Sender API here.
• A Sender instance is now bounded to a particular PushApp, so if we want to send a message to another PushApp we should :
•   Create a new instance of the Sender
•   Expose setters to update the config.

So , wdyt ?