add `UniquePtr::IsValid()` and `UniquePtr::Release()`
FIX: pointer dereference of `UniquePtr`
TEST: unit test set for `UniquePtr`

Conflicts:
	test/unittests/CMakeLists.txt
add `UnlinkGuard` to util.h
TEST: unit test set for `UnlinkGuard`
add `Catalog::GetLastModified()`
add convenience method history db processing
Refactor: extract method `sign_manifest`

Conflicts:
	cvmfs/cvmfs_server
add setters to `Manifest`
allow closure in `Observable::RegisterListener()`
TEST: add closure usage to t_util_concurrency.cc
move HistoryDatabase to history_sql.{h|cc}
add basic implementation of history::History
TEST: basic unit test set for History
add `History::OpenWritable()`
add `History::Remove()`
remove `Sql::IsBusy()` dependency
assert success of `Sql::Init()`
TEST: unit tests for `History::Remove()`
scaffold for `cvmfs_swissknife tag_create`