Warn about outdated/undesired node.js / npm version

[voxpelli]

If we're doing an app for Node 0.10 or for iojs then we want to ensure that everyone that uses that app uses it with the right node version and even more we want to ensure that the tests that are run are run with the right node version.

As different npm versions can also treat package.json files different we also want to ensure that the npm version is fairly new – if not of the latest version so at least one of the third latest or so.

This should be checked on at least push time.