Respect semver requirements in package.json

[schultyy]

engine in package.json can be denoted in semver notation. avm needs to be able to handle that case. For that, Steve Klabnik's semver crate comes in handy. For all node versions >= 4.0.0 it needs to be checked if the denoted version in package.json applies to one of the installed versions on the current system.

All node versions before that need to be treated differently. This needs to be defined.

[schultyy]

For node.js versions < 4 the following rules apply:

• Even minor numbers denote stable versions
• Odd minor numbers denote development versions

[schultyy]

In order to avoid reimplementing a version checker for legacy node versions (< 4.x), autoselect checks for strict equality only. No version ranges etc. are supported