Add index.js so this module can be properly required into projects

Currently the `package.json` [points to a file that doesn't exist](https://github.com/nodesecurity/nsp/blob/master/package.json#L36) with the `main` attribute - the attribute that specifies the entry point when the package is required in programatically. This PR adds the missing index.js file and exposes the two nsp methods on it.