Continuous pinging from browser instead of only login and logout pings.

Previously...

- login ping whenever
-- a gmail tab is opened
-- status is changed from invi / offline to online / busy

- logout ping whenever
-- sign out button is clicked for google account
-- user goes invisible
-- user signs out of chat

Now...

- every minute, an "i am online" ping is made if
  user is busy / avialable / invi
- if the server finds a gap of more than 5 minutes between the pings,
  it assumes that the user has gone offline and accordingly notifies
  offline messages if any.