Update Changelog
Integrate Symfony EventDispatcher

The Symfony EventDispatcher component has been integrated into Sherlock to
help promoted decoupling of components.  To start, the node infrastructure
has been refactored to use the EventDispatcher.

Instead of directly injecting node addresses, Requests now fire an event
which is handled by a separate Cluster object.  The Cluster object then
returns an appropriate, random node.

In the future, this will easily allow retrying nodes on failure, pruning
the node list of inactive nodes, etc.

Further, the dispatch system will make decoupling other components easier,
such as changing the transport layer.

Closes #10
Add instructions for composer dev-master
Merge develop into master