Scope[] now returns nil for undefined variables

Given that we have the 'include?' method, this feature
is unnecessary, and it makes sense to convert to more
ruby-like behavior.

Any code that previously checked whether lookupvar
(or the new []) returned :undefined should now check
whether 'scope.include?(var)'. Thus, you can have
the same behavior, but it takes a bit different code
to get it.

Signed-off-by: Luke Kanies <luke@puppetlabs.com>
Reviewed-by: Nick Lewis <nick@puppetlabs.com>