Change track() and push() API to accept a callback parameter
Refactor building session/visitor/event data urls
Add keepalive option, set max/min limits on ping interval
Implement call() method
Remove visitorData for ping, add sessionData when using track() and push()
Add 'visit' to tracker stub
Update and add new tests for ping and sessionData changes
Stop ping interval when idle is > idle_timeout, instead of not pinging
Update min files