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

data_structures_and_algorithms/trickling-water.py

@@ -1,38 +0,0 @@
-class Node(object):
-    def __init__(self, value, children=[]):
-        self.value = value
-        self.children = children
-
-
-################################################################################
-# Solution
-################################################################################
-def trip_time(node):
-    s = []
-    result = 0
-    s.append((node.value, node))
-    while s:
-        p, node = s.pop()
-        if not node.children:
-            result = max(result, p)
-        for x in node.children:
-            s.append((p + x.value, x))
-    return result
-
-
-################################################################################
-# Tests
-################################################################################
-tree = Node(
-    0,
-    children=[
-        Node(5, children=[Node(6)]),
-        Node(2, children=[
-            Node(6),
-            Node(10),
-        ]),
-        Node(3, children=[Node(2, children=[Node(11)])]),
-    ])
-
-assert trip_time(tree) == 16
-print("Tests pass!")