Add coding exercises for Facebook interviews

Add attempts at solving coding problems to Briefcase.

scratch/facebook/largest-contiguous-sum.py

@@ -0,0 +1,15 @@
+def find_sum(xs):
+    result = float('-inf')
+    streak = 0
+    for x in xs:
+        result = max(result, streak, x)
+        if streak + x <= 0:
+            streak = x
+        else:
+            streak += x
+    return result
+
+
+x = [2,-8,3,-2,4,-10]
+assert find_sum(x) == 5
+print("Success!")