Added shell option to execute command with specific node version in PATH

This allows people to use n to quickly start a shell with a specific version without altering their system:

    $ n shell 0.10.12
    $ node --version
    v0.10.12
    (ctrl+d)
    $ node --version
    v0.10.18

This also helps a lot with installing packages for a specific node version:

    $ n shell 0.10.12 npm install

It all works without write permissions to /usr/local/.