Store last seen on heartbeat every hour

see #2
Add additional documentation
move updating last seen into a function

Hook that function into the admin_init in addition to heartbeat API.  Adds functionality to no-js page loads.
Add last seen time to table

see #2
Merge remote-tracking branch 'aaronjorbin/last-seen'

Conflicts:
	wp-session-manager.php
Inline docs tweaks.
Add some missing i18n.
Use WPSM's session manager method.