(GEP-604) Allow references to preceding definition arguments

This commit ensures that expressions in a definition argument list
are allowed to reference arguments that has been declared previously
in the list and that references to other arguments in the list are
reported as "Reference to not yet initialized variable" rather than
"Unqualified and Unknown variable".
(GEP-113) Include inherited variables when resolving parameters

The specification says that parameter default value expressions
may reference variables defined by the inherited class. This
commit ensures that such references are resolved correctly.
Merge pull request #29 from puppetlabs/issue-604

(GEP-604) Allow references to preceding definition arguments