World: Add Configuration object

World now maintains a reference to an internal configuration
object. World can be configured by executing World.configure(),
which temporarily exposes that object.

Quick finalizes this configuration once all shared examples have been
evaluated.
QuickSharedExampleGroups: Add configure: method

Add a +configure: method to QuickSharedExampleGroups that takes a
Configuration object that can configure Quick behavior.
s/QuickSharedExampleGroups/QuickConfiguration/g

Rename QuickSharedExampleGroups to QuickConfiguration.
Remove +sharedExamples method, leaving only +configure:.