Roles in the RST XHTML visitor did not have their SourceVisitor set

In my project I use the visitor to retrieve objects that are shared between roles and directives. Directives have their setSourceVisitor() method called to store the calling visitor on the directive; Roles also have a setSourceVisitor() method but this is not called during the visit. This commit changes that.
Merge pull request #3 from mvriel/patch-2

Roles in the RST XHTML visitor did not have their SourceVisitor set