LOGBACK-700 + ImportCustomizer for GafferConfigurator

The customizer auto-imports commonly used types, packages and Level constants (incl. their lower-cased names as aliases). That means users won't need to have imports at the top of their logback.groovy files anymore when they only use the common framework-provided types. Also, log levels can be specified in lower case just like in the XML config file format.
The ConfigurationDelegate now has a jmxConfigurator nethod that registers a JMXConfigurator. It takes a single optional String parameter that can be used to pass a custom context name or to replace the entire ObjectName (this seemed nicer than 2 separate parameters from a DSL/API PoV).