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

tools/symlinkManager/README.md

@@ -0,0 +1,14 @@
+# Dotfile Symlink Manager
+
+Find and delete all symlinks to the dotfiles defined in `$BRIEFCASE`.
+
+Oftentimes I corrupt the state of my configuration files. The intention with
+this script is to help me clean things up when this happens. An example workflow
+might look like:
+
+```shell
+> symlink-mgr --audit
+> symlink-mgr --seriously
+> briefcase # changes directory to $BRIEFCASE
+> make install
+```