Remove operation for counters

Allow counters to be removed. If an operation finds out that the counter doesn't exists, it is recreated with its initial value and the operation is replied.

If a getValue is invoked and the counter doesn't exist, it returns its initial value without recreating the counter.

New methods to the API:

CounterManager.removeCounter(String counterName) //remove the counter with this name 
StrongCounter.remove() //removes the counter represent by this instance
WeakCounter.remove() //same as above