Add test and implementation for duplication op
Basic check for empty stack

Check stack is not empty before a pop, throw an error otherwise
Add STACK system command
Added division
Added test for multiplication and addition in the same operation
Dispatch functions for conditions and comments
Implement integer comparison operations
Define Forth conditions
Implement EXIT
Implement VERSION
Added EXIT command
Add Credits for Simon. Proper credits
Dispatch for Forth functions
Forth functions definition
Define Forth comments
Define words NOP and DROP