Ensure dates are properly formatted when overlapping years.

More context was needed than was provided to `ETH_Timeline::format_single_date()`, so determination of whether to show month and year moved to `ETH_Timeline::format_date()` as that method has the necessary context. As a result, `ETH_Timeline::format_single_date()` is now just a helper function for PHP's `date()` command.

Fixes #7.