Added ExtendableInterface for better control over extension registration
Updated README
Implemented basic firewall subscriber
Added security extension and configuration treebuilder
Added initial authenticator implementation
Modified FirewallSubscriber to use SimpleAuthenticator
Added test for SecurityConfiguration
Added SecurityExtension test
Moved SimpleAuthenticator and added test
Fixed docblocks in SecurityExtensionTest
Fixed SecurityExtension config merging
Added initial FirewallSubscriber test
Added AuthenticatorInterface to phpunit exclude