Some small changes, mostly streamlining.

Hi!

I did some small changes:

* The plugin is now called "redmine_reminder" instead of "due_date_reminder". This is reflected everywhere in the code (hopefully).
* The verification regex for the reminder-format has changed and now accepts even empty strings (for no reminders)
* It is possible to receive no reminders by clearing the reminder setting under "my account"
* Some small changes to german and english localization (one new label, only localized to english and german)