Customise 'Arrival date' and 'Departure date'

I have no intention to add a UI for this to the module itself, although I could use it myself as well. So this will either be a dependency on string overrides (for English, normal translation interface otherwise) or the variable module that provides a default interface for changing/setting variables. (Currently, I know of the Colorbox module that does so.)

You can speed this up by:
- creating a list of all strings that may need customisation
- writing a patch that changes all these strings into variables
- extending this patch (or writing a follow-up patch) with an implementation of (all) the hook(s) that the variable module needs.

Corrected incorrectness

I added this feature via the variable module.

If UI texts are missing do not hesitate to inform me.