Implement a dry run for rake install as an argument, plus reorganization

Add:
 * A dry_run argument on the 'rake install' task. Dry run is performed
   as follows: rake install[true]

Refactor:
 * Organization of installations and symlinks
Update Rakefile so it can be re-ran without .backup files

This change comes from codyleblanc/dotfiles (76de6c3ab5)
Update global gitignore file (general things to ignore)