Added getSubscriptionsCount function to KnockoutComputedFunctions

This function was missing and is expected to be available to all KnockoutComputer<T> objects, as stated in the docs:
http://knockoutjs.com/documentation/computedObservables.html#computed_observable_reference
Merge pull request #794 from jfoliveira/master

Added getSubscriptionsCount function to KnockoutComputedFunctions