Fixed word precedent.
Changed equality operator from single = to double ==.
Fixed unit test to reflect change.
Added variable assignment, recall and scoping.
Added associated unit test methods.
Added extra test methods for variable scoping.
Fixed a minor error in D4ScriptEngineTest.testVarNotDef().
Combined return stack and scope stack to a single ActivationRecord
stack.
Also some general cleanup.
Refactored tryCoreWord() to fully segment core words into general type
groups.