Add coding exercises for Facebook interviews

Add attempts at solving coding problems to Briefcase.

scratch/facebook/stack.py

@@ -0,0 +1,25 @@
+class Stack(object):
+    def __init__(self):
+        self.items = []
+
+    def __repr__(self):
+        return self.items.__repr__()
+
+    def push(self, x):
+        self.items.append(x)
+
+    def pop(self):
+        if not self.items:
+            return None
+        return self.items.pop()
+
+    def peek(self):
+        if not self.items:
+            return None
+        return self.items[-1]
+
+def from_list(xs):
+    result = Stack()
+    for x in xs:
+        result.push(x)
+    return result