add history::TagList::GetReferencedHashes()
CatalogTraversal<> can traverse named snapshots
TEST: create dummy history over dummy catalogs
TEST: CatalogTraversal<>::TraverseNamedSnapshots()
FIX: assertion prevents from (allowed) double Traverse()
Merge pull request #500 from reneme/feature-traverse_history

Feature: CatalogTraversal<> Traverses all Named Snapshots

This adds CatalogTraversal<>::TraverseNamedSnapshots() which traverses all revisions referenced in the repository's history database.

Additionally it adds test cases in T_CatalogTraversal to verify the functionality using a mocked history.