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/plot.py

@@ -0,0 +1,9 @@
+import numpy as np
+import matplotlib.pyplot as plt
+
+rng = np.random.RandomState(10)  # deterministic random data
+a = np.hstack((rng.normal(size=1000), rng.normal(loc=5, scale=2, size=1000)))
+_ = plt.hist(a, bins='auto')  # arguments are passed to np.histogram
+plt.title("Histogram with 'auto' bins")
+Text(0.5, 1.0, "Histogram with 'auto' bins")
+plt.show()