implement a method_missing in the Binding class #1

This will catch references to non-existent variables and bubble them up with a
useful line number.

    $ puppet apply --modulepath spec/fixtures/modules -e 'include issue1'
    Error: /Stage[main]/Issue1/Datacat[/tmp/issue1.1]/Datacat_collector[/tmp/issue1.1]: Could not evaluate: Could not find value for 'scope' issue1/refers_to_scope.erb:7