Piwik.js JS tracker: allow developer to pass a 'callback' argument

Implemented in the API #trackLink method, as it’s the one in which using callback makes most sense but it can be added to all of the other methods as well.

Somewhere in the code where link `click` is being caught:
`_paq.push(['trackLink', url, type, {}, function() { window.location = url }])`

In this way we don't block the UI in `#beforeUnloadHandler` function.