Added methods to set the Visitor Id in the Tracker class

In order to be able to connect events from a client (Webclient, Android,
iOS) with events created by a backend component (based on some
interactions from the client with the backend), it is very helpful
if the Visitor ID can be set to a specific value.
Therefor methods were added to the Tracker class with which the Visitor
ID can be set and also verified that the Visitor ID format is exactly 16
bytes long and only containing lowercase hexadecimal characters.
Added 'link' key to the Query Parameter Constants

In order to create consistent events from different clients (Webclient,
Android, iOS) it can be useful to set the 'link' value.
Therefor I added a new Query key which can be used with the public
'set' method of the tracker in order to provide a 'link' Uri along with
the event.
Merge pull request #13 from artcom/master

Added support to set Visitor ID and link value