FIX: compile error due to dependent base class lookup failure
Merge pull request #490 from reneme/fix-compiler_error

FIX: Compile Error due to Dependent Base Class Lookup Failure
multiple Traverse() calls for one CatalogTraversal object
pre-define root catalog hashs in T_CatalogTraversal
TEST: verify multi traversal feature of CatalogTraversal<>
Merge pull request #491 from reneme/feature-multi_clg_traversal

Feature: CatalogTraversal<> Objects can be Reused with Multiple Entry Points

The CatalogTraversal<> template can now be reused for multiple traversal operations. This will become handy, when it is used for the garbage collection of a repository with multiple tags. Hence each tag will be an entry point into the repository to be garbage collected.

Additionally this contains an adaption/extension of the respective test suite.
add scaffold for `cvmfs_swissknife gc`
factor out MockCatalog for code reuse
add mocked files to MockCatalog
add file chunk mocks to MockCatalog
TEST: scaffold for garbage collector test case