STORM-512 KafkaBolt doesn't handle ticks properly

I implemented this by 
1.   Adding an 'boolean isTick()' method to the Tuple interface. 
       This way it is for application builders much easier to  determine if the tuple at hand is a Tick or not.
2.   Letting the KafkaBolt use that method to ignore the ticks.
3.   Updating the other place in the code base that handles ticks differently to use this new method.