only install curl if it isn't defined elsewhere

If ensure_package() is used with a conventional resource declaration
such as `package { 'curl': ensure => present, }`, it could result in
a duplicate resource error. This commit ensures that curl is not
declared if it's already defined elsewhere, such as a site-wide packages
class.
Merge pull request #39 from rji/fix-curl-resource

only install curl if it isn't defined elsewhere