Implement a stack such that push(<item>), pop() and
getMinimumValue() have an amortized complexity of O(1).
push(<item>) and pop() have the usual implementation.
getMinimumValue() returns the minimum value currently stored
in the stack.
For example:
stack.push(5)
stack.push(3)
stack.push(27)
stack.getMinimumValue() => 3