add `UnlinkGuard` to util.h
add `Catalog::GetLastModified()`
scaffold for `cvmfs_swissknife tag_create`
add `UniquePtr::Release()`
TEST: unit test set for `UniquePtr`
FIX: pointer dereference of `UniquePtr`
TEST: unit test set for `UnlinkGuard`
add convenience method history db processing
add `History::SetPreviousRevision`
add `History::Exists()` and `History::Get()`
TEST: adapt history unit test cases
revise `cvmfs_swissknife tag_create`