Update Table::get('row') to better check for right offset
Add groups (head, body, foot) for Tables
Add missing $_curr_cell to \Table\Group
Add missing interfaces Countable and Iterator to \Table\Group
Add \Table\Row
Change \Table\{Group,Row}::unset to \Table\{Group,Row}::clear
Add fully working, simple usage package content

Next up: Add config and support everything that's in TODO (pagination, filtering, sanitizing, ...)