Added 'link' key to the Query Parameter Constants
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.

piwik/piwik-sdk-android,d4rken/piwik-sdk-android,hannesa2/piwik-sdk-android,xmaximin/piwik-sdk-android,eldk/piwik-sdk-android,seancunningham/piwik-sdk-android