Shell script fixups and POSIX standard compatibility

This supersedes #32 - *Assimp3* install and POSIX standard compatibility.

What began as a fix-up to our *assimp2* uninstaller and *assimp3* installer stanzas expanded into making our shell scripts as cross-platform compatible as possible. Plus a few other updates along the way.

The goal is to have the scripts run sucessfully on anything that ```/bin/sh``` symlinks to. For example, ```dash``` in Debian/Ubuntu.

Two important resources were Canonical's [guide for porting from bash to dash](https://wiki.ubuntu.com/DashAsBinSh) and [using the shellcheck linter in Sublimetext](https://github.com/SublimeLinter/SublimeLinter-shellcheck)