Add implementation for the Envelope, Message, Response & User
Merge pull request #15 from laurentkempe/feature/envelope

Add implementation for the envelope
Split responsibility of TeamCityListener into TeamCityAggregator

TeamCityListener is listening to Http post from a TeamCity server then Emit a message which TeamCityAggregator listen to and send a notification.
Handle TeamCity builds to output only one success, failure message
Notify in case of build error
Fix Rx way to subscribe to the group of builds
Use Rx Take() operator instead of CancellationToken
Adapt to new config per plugins
Add TeamCity Build Trigger plugin
Fix path building for plugins views
Rename IMessenger to IEventEmitter to avoid confusion