jMe - Handling invalid expressions
jMe - ExpressionString understands if its valid or not
jMe - Unnecessary double quotes.
jMe - ExpressionString model now understands valid expression strings and provides a method to iterate over the expression string
jMe - Validating the max allowable expression string length
jMe - Refactoring to extract a stack model that understands raises exception if you pop an empty stack
jMe - Minor refactoring in specs
jMe - Handling overflow of 12 bit integers