Added a HasSubscription() method
Mostly implemented Subscribe()
Implemented most of Unsubscribe()

The implementation seems a bit unoptimal since it doesn't track where the
handler was passed to.
Started on a subscription class for tying sub IDs to sessions
Added subscription tracking
Added EventTriggered method to (un)subscribe
Added the structure for a subscription event message
Added message ID for SubscriptionEventMessage
Changed accessibility to public
Implemented EventTriggered

A subscription event message is now sent to the remote client using the
session.
Added an event args ID

This is needed when unmarshaling the subscription event args data.
Added properties for the event args
Force _marshaledEventData to never be null
Implemented marshal functionality
Code the handoff from the subscription message
Added a class for constructing objects from their byte representation
Implemented functionality
Added a TryConstruct() method
Implemented ProcessSubscriptionEventMessage

Handlers (subscribers) are now called when a remote event is received.
Added a register method

This is needed so that sub-types of SubscriptionEventArgs can be
unmarshaled.