Enable row editing by default

According to [the documentation][1] in `UITableView.h`, if a `UITableViewDataSource` doesn't implement `-tableView:canEditRowAtIndexPath:`, “all rows are assumed to be editable”.

In SSBaseDataSource’s implementation, the core logic is to rely on the `fallbackTableDataSource` if it implements the method, but otherwise return a sane default. I think the sane default should mimic the behavior of not implementing the method at all, that is, it should simply return `YES`.

[1]: http://cl.ly/image/3i331s1I0G3e