Modified the Instance class adding 2 attributes.

Modified the KNN class adding a PriorityQueue for sorting. The only
thing left is getting the local mode among the K nearest neighbors and
resolving the conflicts. For example there is no a clean local mode. Or
if we want K = 1 if there are more than one instances at the same
distance, meaning that there is no nearest neighbor.

Added A JUnit test for debugging and automated testing.