[WIP] Proposal for property change listeners on client objects

[MarkTwoFive]

Introduces property change listeners on special IDataHolder instances.
Proposed implementation:

• Synchronously run listeners directly after the property has been changed.
• Defer running listeners during client cache update and run them after the update is completed
• Document that changing properties in an update listener is considered an anti-pattern but do not enforce.
• TODO: Document that listeners should not be invoking any sort of load bearing code due to their synchronous nature.
• TODO: Write documentation about this feature on ReadMe.IO

Closes: #95

[MarkTwoFive]

@Genius42 please review the proposed API addition.