install nodejs and npm on gentoo
Merge pull request #71 from hairmare/feature/gentoo

install nodejs and npm on gentoo
Give option to name nodejs and npm package name by creating variables $node_package_name and $npm_package_name in class definition.

Give option to ignore npm package with install_npm_package flag.

Example:
```
class { '::nodejs':
    install_npm_package => false
}
```
Updated documentation