Heartbeats working

The extension now has heartbeats and they work like this:

• every 2 minutes if there is activity in the browser a heartbeat is sent
• every time an active tab is changed a heartbeat is sent
• every time a tab is updated it checks id the updated tab is also an active tab and then it sends a heartbeat

A heartbeat is composed of:

• the current active tab URL or only domain (the user can specify in the options)
• the type of the URL (url or domain)
• current UNIX timestamp
• a boolean value indicating if the devTools are open (currently this does not work, always returns false)