Use a default file if it exists, otherwise fallback to nom's default

Another attempt at resolving issue #10 

Instead of having a `~/.npmrc.bak` file to use if no `.npmrc` is found, it will check to see if a default npmrc file exists under the name `~/.npmrc.default` and use that. If a default isn't found, it will just delete the old `~/.npmrc` file that was being used and fallback to the default settings that NPM uses (This is located wherever NPM is installed).