Update osx dotfile.

- Specify OS X Mavericks in the comments.

- Update comments to include how to undo each setting (where possible).

- Disable internal MacBook display while connected to external monitor
  but while having the MacBook remain open.

- Disable startup chime.

- Change LocalHostName and / or ComputerName if ~/.localhostname and /
  or ~/.computername exists with the values LOCALHOSTNAME and / or
  COMPUTERNAME, respectively.

- Restart Finder at the end to ensure all settings are updated
  immediately.