Basic specs
Basic Stack and StackMachine implementations
More complex tests: Basic operations and overflow/underflow specs
StackMachine implementation
Added spec for empty string
Refactored stack_machine_emulator to pass the test
Checked storage on 12 bit unsigned ints on the stack
Added checks for overflow/underflow
Added specs for it both on Stack and StackMachine
Refactored error messages
Checked on data type on Stack#push, now it will only accept integers