Added system message action and rules action interface.
Added a test case for optional context values for the show message action.
Fixing yet another typo in ContextAwarePluginBase.
Fixed comment and ID of system message action.
Merge pull request #46 from klausi/system-message-2269587

Added system message action and rules action interface.
Refining @return statements in ConditionTestBase.
Renamed ActionExpression to RulesAction and ConditionExpression to RulesCondition.
Converted all ActionInterface instances to RulesActionInterface.
Merge pull request #50 from klausi/action-context

Covert all ActionInterface instances to RulesActionInterface.
Fixing unit tests.
Clean up the documentation blocks.
Merge pull request #60 from bartfeenstra/8.x-3.x

Clean up the documentation blocks.
Converted PathAliasExistsTest to unit test.