Allow missing calendar.txt data.

Fixes #562.

* Fixes scala feed date range endpoint check to try calendar_dates, too
* Fixes sample period validity check to try calendar_dates, too
Set up buffered pruning of stops to allow a window beyond the sample period
Merge pull request #563 from moradology/feature/buffer-stop-pruning

Set up buffered pruning of stops to allow a window beyond the sample period
Merge pull request #564 from flibbertigibbet/feature/allow-missing-calendar-txt

Allow missing calendar.txt data.