Added:
	- An StopWatch class to keep track of the execution time.
	- A new custom Exception to check if the number of clusters is greater
than the number of instances.
	
Updated:
	- The behavior when a cluster has no instances associated. Now when no
instances are associated the cluster remains unchanged. Before the
centroid new attributes was an empty list
Merge branch 'K_Means_Clustering' of https://github.com/RubenAgudo/MineriaDatos.git into K_Means_Clustering