Switch from /bin/sh to /bin/bash

You have bashisms in your script and /bin/sh might not always be bash. In that case the script would fail (for example on Ubuntu).
Remove unneeded shebang in nvm.sh
Merge pull request #270 from hachre/patch-1

Switch from /bin/sh to /bin/bash
Unset the executable bit on nvm.sh, since it's meant to be sourced
Adding a CONTRIBUTING.md file
nvm exec wrapper to execute commands with a specific node version