contactBegin always fires

[karmacod3r]

The "contactBegin" event keeps firing on every step on continues contacts.

I tracked it down to TupleDictionary.set: the key should be added to the keys list when it is NOT already in it:

    if (! this.data.containsKey(key)) {
      this.keys.add(key);
    }

In the current version the contacts are not stored properly and so appear as new contacts on every step.

Sorry, I'm new to Github - didn't know if this would be better in a pull request.

Keep up the good work!