Discover newer versions of dependencies than package.json allows

The reference issue is https://github.com/isaacs/npm/issues/1428

Newer packages are looked for when issuing a npm outdated on the project.
If a newer package is discovered then there are two options, basing on the package.json:
 * the newer is not allowed, in the message OUTDATED is showed
 * the newer is allowed, in the message CANUPDATE is showed 

In both cases the newer version discovered is showed.

Let me know if I made any kind of mistake here, such as this is not what was expected or if there are bugs I've not spotted.

Cheers.