Implement support for Variadic arguments

Fixes https://github.com/phpDocumentor/phpDocumentor2/issues/629, in
this commit we add support for recognizing and displaying variadic
arguments as described in the Variadics RFC on Internals
(https://wiki.php.net/rfc/variadics).

This adds support for describing Variadics even before PHP 5.6, where
this feature is planned.
Merge branch 'master' of github.com:phpDocumentor/ReflectionDocBlock