Add the setup.bash script, which installs dotfiles

The setup.bash script creates symbolic links from the individual
dotfiles in this repo to the $HOME directory. Before doing so, any
existing files that would be overwritten are saved in directory called
`backup_dotfiles`. Any existing symbolic links are removed.

The .gitignore file was added to ignore the `backup_dotfiles` directory
and any Vim swap files.
Re-group bash/zsh settings and update setup.bash

A `shell` directory was created with sub-directories for bash and zsh
files. The `.bash_profile`, `.bashrc`, and `.zshrc` files were moved to
their appropriate new sub-directories.

As a result, the `setup.bash` script was updated to use the new file
locations when creating symbolic links.