Implement suspect/resumeDelivery()

[rafaelw]

Moved from: https://code.google.com/p/mutation-summary/issues/detail?id=19

https://groups.google.com/forum/?fromgroups=#!topic/mutation-summary-discuss/olsh5jJt7t8

The idea is that clients may want to only poll for changes on their timeline. suspendDelivery() will accumulate changes and all will be reported when takeSummaries() is called. resumeDelivery() would resume end-of-microtask delivery of changes.