Rearrange PODs. Most of the spec of StatusStorage is now moved to Timeline. This is because Timeline is nearer to end-users, and it should be as self-contained as possible. POD of StatusStorage is for storage implementors, which would be very limited.
Timeline: ids argument for delete_statuses() is now mandatory. If it's optional, trivial typo such as 'id => 20' destroys all statuses in a timeline, which is very dangerous. Add test_storage_missing_arguments() test to Test..StatusStorage.pm to test missing mandatory parameters.
t/Timeline.t: write tests for status methods.
Timeline: implement status methods. Pass the tests.
t/Timeline.t: write tests for get_unacked_counts().
t/Timeline.t: add tests in which StatusStorage returns errors.
t/Timeline.t: add tests for filters.