Make avm cross platform

[schultyy]

Right now, avm cannot run on Windows because it relies on the tar command for unzipping downloaded archives.
This needs to be replaced with a mechanism which works on all platforms without problems.

[skade]

https://github.com/alexcrichton/tar-rs/ is a pure Rust implementation of tar.

[schultyy]

Also there's no package for Windows which contains both npm and nodejs. There's only an MSI Installer and a nodejs standalone package without npm

[schultyy]

This might do the job: https://gist.github.com/manuelbieh/4178908
One problem is there though: avm needs admin privileges in order to be able to install something new node versions

[schultyy]

To decompress gzip archives this here seems helpful: https://github.com/alexcrichton/flate2-rs