added npm.pp from puppetlabs-nodejs and make_default option

Hi, I just switched from puppetlabs-nodejs to willdurand/puppet-nodejs to get the latest node version installed with my puppet-sinopia module and to get out of package manager hell for nodejs+npm installations on debian systems for now. Unfortunately, saheba/puppet-sinopia relies on the npm.pp from puppetlabs. I merged npm.pp from puppetlabs-nodejs into your puppet nodejs module, so I can enjoy the best of both ;-) I also added a make_default option to be able to select one nodejs installation as the primary one by adding a symlink /usr/local/bin/node pointing to the version which is installed with make_default to true. This change is backwards compatible, because the option's default value is false.

The rake test run results for this branch:
---> syntax:manifests
---> syntax:templates
---> syntax:hiera:yaml
Cloning into 'spec/fixtures/modules/stdlib'...
remote: Counting objects: 5561, done.
remote: Total 5561 (delta 0), reused 0 (delta 0)
Receiving objects: 100% (5561/5561), 1.10 MiB | 423 KiB/s, done.
Resolving deltas: 100% (2307/2307), done.
Cloning into 'spec/fixtures/modules/wget'...
remote: Counting objects: 534, done.
remote: Compressing objects: 100% (262/262), done.
remote: Total 534 (delta 223), reused 534 (delta 223)
Receiving objects: 100% (534/534), 82.21 KiB, done.
Resolving deltas: 100% (223/223), done.
/.rvm/rubies/ruby-2.1.2/bin/ruby -S rspec spec/classes/nodejs_spec.rb spec/defines/nodejs_install_spec.rb spec/functions/is_binary_download_available_spec.rb spec/functions/is_npm_provided_spec.rb --color
WARN: Unresolved specs during Gem::Specification.reset:
      rake (>= 0)
      rspec (>= 0)
WARN: Clearing out unresolved specs.
Please report a bug if this causes problems.
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Finished in 3.57 seconds
47 examples, 0 failures

Let me know if there is anything else to do to get this merged.