Shebang or supported shells

Current shebangs in source files (`#!/bin/sh`) are wrong.

`#!/bin/sh` means that you're using [POSIX shell](http://pubs.opengroup.org/onlinepubs/009695399/utilities/xcu_chap02.html), but you don't (for example: `local`, `[[`, wildcards in tests, arrays - they are bash specified). I don't mean that it's bad - real POSIX shell scripts are rather obscure and making them portable may be [really hard](https://www.gnu.org/software/autoconf/manual/autoconf.html#Portable-Shell). So easiest way is change shebangs to `#!/usr/bin/env bash`.

And users of other shells always could try to run these scripts explicit, eg. `$ zsh nv`.