VariablesSystem renamed to VariablesManager
VariablesManager is not a System anymore.
Also: some outdated javadoc fixed along the way
VariablesManager added as a field of EngineApplicationListener that is passed down to GameLoader, EngineInitializer, etc.
Accommodating API changes (review not needed)
Package ...engine.systems.variables moved to ...engine.variables (review not needed)
Accessor embedded in VariablesManager
Merge pull request #437 from jtorrente/variablesManager

Variables manager - Fixes #394