Prevent leak from computes that have no bindings.

If the same event triggers an update to a compute AND
the same event also unbinds the compute, it can result in
the compute having bindings = 0, but still haveing a bound
onchanged handler.

This results in unecessary recomputes and can leak Models in
the Model store.
Merge pull request #310 from Spredfast/unbound-compute-leak

Prevent leak from computes that have no bindings.