Ndenv_npm does not install package

Seems packages aren't installed when there's no version specified,
so after looking the source of the `chef/provider/package.rb`
file (https://github.com/opscode/chef/blob/master/lib/chef/provider/package.rb#L59),
if there is no version the value of attribute :version must be
nil instead of empty string.
Update readme
Improve tests by adding a fake recipe for kitchen
Remove build-essential warnings

Fix this warning :

WARN: A build-essential recipe does not exist for 'chefspec'. This
means the build-essential cookbook does not have support for the
chefspec family. If you are not compiling gems with native
extensions or building packages from source, this will likely not affect you.
Add test for source parameter
Rename fake recipe to fake-ndenv
Check bower version in serverspec
Merge pull request #2 from PierreRambaud/master

Ndenv_npm does not install package