change google analytics interface

This is a related to my post at https://groups.google.com/forum/?fromgroups#!topic/gwt-platform/0cfnA9om3DM

It is a breaking change so needs thorough review before being accepted.

I removed the binding from default module since you now have to bind userAgent at the same time.  I think it might be good to add a separate analytics gin module so that analaytics, navigation tracker & UA can be installed all at once.

I moved the GAAccount annotation to the google analytics folder because I think it's a mistake to put all annotations in the same package just because they're annotations.  GAAccount.java is much more related to GoogleAnalytics.java than it is to DefaultPlace.java
Sorry my eclipse adds finally to everything automatically.  The reason to add it to methods is it shows instantly that the parameter won't be modified.  But I'm happy to remove it if it bothers people.