Update bin/dotfiles variables

Update `DOTFILES_GIT_REMOTE` and `DOTFILES_TARBALL_PATH` variables to
refer to the current repository.
Update scripts to improve portability

Update the shebang from `#!/bin/bash` to `#!/usr/bin/env bash`.
Rewrite shell/bash_prompt

Rewrite based on @mathiasbynens’s prompt. Rename `prompt_git` function,
`branchName` and `s` variables to improve readability. Remove the
working directory from the Terminal title.