Fix bug related to coordination numbers

When deducing the century of a coordination number the normalized form must still be a coordination number. I.e., the day component is still increased by 60.

An example from the test cases (with a reference date of 2017-12-12): the coordination number 171270-0002 should normalize to 20171270-0002. Before this fix it was instead normalized to 20171210-0002.

A note on getting started

Just an update to the README on how to use the library. The bare minimum really.

Expose parse errors

Expose parse errors so that callers can tell different errors apart.

Hello, World!

Feature complete, i.e, should handle all Swedish personal identity numbers thrown at it. Needs a tad more documentation.