Add coding exercises for Facebook interviews

Add attempts at solving coding problems to Briefcase.

scratch/facebook/recursion-and-dynamic-programming/permutations.py

@@ -0,0 +1,13 @@
+def char_and_rest(i, xs):
+    return xs[i], xs[:i] + xs[i+1:]
+
+# perms :: String -> [String]
+def perms(xs):
+    if len(xs) == 1:
+        return [xs]
+    result = []
+    for c, rest in [char_and_rest(i, xs) for i in range(len(xs))]:
+        result += [c + perm for perm in perms(rest)]
+    return result
+
+print(perms("cat"))