Make `addData(WithObject)` *add* data (not overwrite) about node-gcm

[hypesystem]

This would be a breaking change... maybe we should add a branch for breaking changes that will be merged into master at 1.0?

[hypesystem]

it would make sense if there was also a set function doing pretty much what we do now, but for any value:

//Overwrite data field
message.set("data", {
    knownValue: "atSomePoint",
    otherValue: "theseOverwriteEverythingPreviouslySet"
});

//Set new value for one of the parameters sent to gcm
message.set("collapseKey", "newCollapseKey");

[hypesystem]

If data gets its own addData method, notification should have the same:

message.addNotificationField("title", "Hello, World");