Replaces the old thread-unsafe MonitorEvents API with a new Monitor extension method that will return a thread-safe monitoring scope that exposes methods like Should().Raise() and metadata such as OccurredEvents and MonitoredEvents.

• Have EventMonitor use a static map
• Ensure EventMonitor.registeredRecorders is thread-safe
• Make EventRecordsMap thread-safe
• Have IEventMonitor implement IDisposable so that it unsubscribes
• Rewrite ShouldRaise as Should().Raise() by moving it to ReferenceTypeAssertions.
• Do we need to support nested scopes?
• Add the possibility to get the metadata for the monitored events
• Add the possibility to get the occurred events

Closed #477 #348