Handle unknown air dates better

Air dates for upcoming episodes, that only specify the year or the year
and the month are now shown with the available info. No assumptions
about the missing day and/or month is made, which means the "Days
remaining" field will show "N/A".

Fixes #2.
Merge pull request #5 from SteffenBach/approx-air-date

Handle unknown air dates better - Closes #2.