Abstract event registry to allow for namespacing.

This allows for multiple event registries that can contain the same
event names, but be executed in separate handlers. The `EventManager`
remains backwards by moving all event management functionality to the
`EventRegistry` class and keeping a single static "global" event
registry object to perform the methods.
Updated readme example to use FQN.
Fixed backwards compatibility for priority constants.
Merge pull request #15 from cjsaylor/event-registry-namespacing

Abstract event registry to allow for namespacing.