nbd relative module paths

Instead of using fully qualified `nbd`, we use relative paths inside the
library itself so that it can resolve without a paths declaration or
having to npm install the module.
Updated package.json

Removes the ./node_modules relative paths to call executables in
dependencies.

Adds the promises-aplus-tests to npm test script
Merge pull request #36 from Aintaer/master

Relative module paths