Portable bash shebangs

This change makes bash scripts more portable by using `#!/usr/bin/env bash` shebangs instead of just `#!/bin/bash`, which only is common in GNU/Linux systems, but not other Unix-like systems. Currently supported systems will most likely remain completely unaffected by this change.

An example of such an operating system is FreeBSD (see #630), which doesn't come with bash per default, but requires it to be installed from the ports collection. It otherwise appears to work pretty good (in fact I am working on porting it there currently. It compiles already).

See [Wikipedia](http://en.wikipedia.org/wiki/Shebang_%28Unix%29#Portability) for further reference.