added bash script to check correct environment before installing

I got tired of the many problems people have installing node-oracle, because of the necessary preliminary steps (and RTFM).
I created a script that checks for the most common errors (e.g. links to non-existing files) and tried to automate most of the steps. I consider this script alpha quality. I have tested it on OSX (bash&zsh) and CentOS (bash only).
My system has libaio installed, so I did not test the code for installing libaio.