Splice ./universe directory into ./

Manually merging:
- README.md: I added the description from universe/README.md into the heading of
  dotfiles/README.md.
- .envrc: dotfiles/.envrc was a superset of universe/.envrc
- .gitignore: Adding some of the ignored patterns from universe/.gitignore to
  dotfiles/.gitignore

Everything else here should be a simple rename.

data_structures_and_algorithms/perm-tree.py

@@ -0,0 +1,83 @@
+import unittest
+
+
+################################################################################
+# Answer
+################################################################################
+class Node(object):
+    def __init__(self, value, children=set()):
+        self.value = value
+        self.children = children
+
+
+# treeify :: Char -> Set(Char) -> Node(Char)
+def treeify(x, xs):
+    return Node(x, [treeify(c, xs - {c}) for c in xs])
+
+
+# dft :: Node(Char) -> [String]
+def dft(node):
+    result = []
+    s = []
+
+    s.append(('', node))
+
+    while s:
+        p, n = s.pop()
+        p += str(n.value)
+
+        if not n.children:
+            result.append(p)
+        else:
+            for c in n.children:
+                s.append((p, c))
+
+    return result
+
+
+# main :: String -> Set(String)
+def get_permutations(xs):
+    if xs == '':
+        return set([''])
+
+    ys = set(xs)
+    trees = []
+
+    for y in ys:
+        trees.append(treeify(y, ys - {y}))
+
+    result = set()
+
+    for t in trees:
+        for d in dft(t):
+            result.add(d)
+
+    return result
+
+
+################################################################################
+# Tests
+################################################################################
+class Test(unittest.TestCase):
+    def test_empty_string(self):
+        actual = get_permutations('')
+        expected = set([''])
+        self.assertEqual(actual, expected)
+
+    def test_one_character_string(self):
+        actual = get_permutations('a')
+        expected = set(['a'])
+        self.assertEqual(actual, expected)
+
+    def test_two_character_string(self):
+        actual = get_permutations('ab')
+        expected = set(['ab', 'ba'])
+        self.assertEqual(actual, expected)
+
+    def test_three_character_string(self):
+        actual = get_permutations('abc')
+        expected = set(['abc', 'acb', 'bac', 'bca', 'cab', 'cba'])
+        self.assertEqual(actual, expected)
+
+
+unittest.main(verbosity=2)