Bugfix: AgentID is made immutable to avoid shared AgentID owners being changed by other agents
GroovyAgentExtensions renamed to GroovyExtensions, and reimplemented using metaClass rather than mixins

This allows more flexibility in adding methods to different classes rather than a single Agent class
Also added addXXXBehavior methods for convenience
Enabled GroovyExtensions globally whenever GroovyBoot is used