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.

Fixes #8. Closes #8.

Fixes #5. Closes #5.

Better handling of empty dates, showing an empty field rather than the Gregorian equivalent of the Unix epoch.

See #3.

Taken at http://git.ethitter.com/wp-plugins/speaking-appearances/commit/7ba1e6ce8d37d69214da8f11270228b61bf2d2c1.