added name to metadata.rb
Don't install NPM if already installed

Installing the latest npm from source installs the npm binary
to /lib/node_modules/npm/bin/npm-cli.js and creates a symlink in
/usr/bin/npm (not in the nodejs install directory)
Updated default npm version to latest

updated to 1.3.15 as 1.1.0-3 no longer available athttp://registry.npmjs.org/npm/-/npm-1.1.0-3.tgz
Fix package existance checks

On my chef client, the npm_package executes were always firing, even
when the npm package was already installed.

This simplifies the `not_if` and `only_if` checks.
Fix package existance check for versioned packages