enable hardlink stresstest on ubuntu
TEST: enable aufs exdev rename on ubuntu
Merge pull request #590 from reneme/test-enable_hardlink_test

TEST: Enable Hardlink Stresstest on Ubuntu
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`
TEST: unit test set for `UniquePtr`
add `History::SetPreviousRevision`
add `History::Exists()` and `History::Get()`
TEST: adapt history unit test cases
revise `cvmfs_swissknife tag_create`
Refactor: `CommandCreateTag` for code reuse
add `cvmfs_swissknife tag_remove`
add assertion for upload