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/inflight-entertainment.py

@@ -0,0 +1,35 @@
+# possible :: Int -> [Int] -> Bool
+def possible(flight_duration, film_durations):
+    seeking = set()
+
+    for x in film_durations:
+        if x in seeking:
+            return True
+        else:
+            seeking.add(flight_duration - x)
+
+    return False
+
+
+should = [
+    (10, [1, 9, 8, 8, 8]),
+    (10, [1, 9]),
+    (10, [1, 9, 5, 5, 6]),
+    (1, [0.5, 0.5]),
+    (1, [0.5, 0.5]),
+]
+
+for a, b in should:
+    print("Testing: %s %s" % (a, b))
+    assert possible(a, b)
+
+shouldnt = [
+    (10, [1, 10, 1, 2, 1, 12]),
+    (1, [0.25, 0.25, 0.25, 0.25]),
+    (5, [1, 2, 2]),
+]
+for a, b in shouldnt:
+    print("Testing: %s %s" % (a, b))
+    assert not possible(a, b)
+
+print("Tests pass")