Bug in bst_validate solution

The solution for [ bst_validate](http://nbviewer.jupyter.org/github/donnemartin/interactive-coding-challenges/blob/master/graphs_trees/bst_validate/bst_validate_solution.ipynb) won't work if the tree has a node with the value of -sys.maxsize.

A suggested tweak would be to use None instead of sys.maxsize and -sys.maxsize.

```python
class BstValidate(Bst):

def validate(self):
if self.root is None:
raise TypeError('No root node')
return self._validate(self.root)

def _validate(self, node, minimum=None, maximum=None):
if node is None:
return True

if not ((minimum is None or node.data>minimum) and (maximum is None or node.data<=maximum)):
return False

if not self._validate(node.left, minimum, node.data):
return False

if not self._validate(node.right, node.data, maximum):
return False

return True
```

I would be happy to prepare a PR if this makes sense to you.