Fix Rx expression to properly handle build timeouts in TeamCityAggregator

Rx Buffered observable needed to be unsubscribed after the first buffered event was received (either because of count or timeout) otherwise we kept receiving empty "timeout" events.
Use Rx Take() operator instead of CancellationToken
Merge pull request #8 from lionelplessis/feature/events

Fix Rx expression to properly handle build timeouts in TeamCityAggregator...