shell: `pip-app` works equally well with Bash

The shebang is not necessary when sourcing a file, only when executing
it.  `pip-app` works equally well with Bash and by skipping the shebang
it can be used on a machine where `zsh` is unavailable.
Merge branch 'feature/bash-support' of https://github.com/dholm/pip-app

Conflicts:
	pip-app.sh
Fixed license link.