Tidy up structure of briefcase

I had a spare fifteen minutes and decided that I should tidy up my
monorepo. The work of tidying up is not finished; this is a small step in the
right direction.

TL;DR
- Created a tools directory
- Created a scratch directory (see README.md for more information)
- Added README.md to third_party
- Renamed delete_dotfile_symlinks -> symlinkManager
- Packaged symlinkManager as an executable symlink-mgr using buildGo

scratch/data_structures_and_algorithms/dijkstra-shortest-path.py

@@ -0,0 +1,48 @@
+from collections import deque
+from heapq import heappush, heappop
+from fixtures import weighted_graph
+
+
+def put(t, x, xs):
+    if t == 'stack':
+        return xs.append(x)
+    if t == 'queue':
+        return xs.append(x)
+    if t == 'priority':
+        return heappush(xs, x)
+
+
+def pop(t, xs):
+    if t == 'stack':
+        return xs.pop()
+    if t == 'queue':
+        return xs.popleft()
+    if t == 'priority':
+        return heappop(xs)
+
+
+# shortest_path :: Vertex -> Vertex -> Graph -> [Vertex]
+def shortest_path(a, b, g):
+    """Returns the shortest path from vertex a to vertex b in graph g."""
+    t = 'priority'
+    xs = []
+    seen = set()
+    # Map(Weight, [Vertex])
+    m = {}
+
+    put(t, (0, [a], a), xs)
+
+    while xs:
+        w0, path, v = pop(t, xs)
+
+        seen.add(v)
+        if v == b:
+            m[w0] = path
+        for w1, x in g.get(v):
+            if x not in seen:
+                put(t, (w0 + w1, path + [x], x), xs)
+
+    return m
+
+
+print(shortest_path('a', 'f', graph_a))