Remind, Abook and multi storage adapters

Remind:
- Storage adapter for the remind calendar software:
  http://www.roaringpenguin.com/products/remind
- Needs python-vobject library.
- Config file options: storage.remind_file, storage.multi_user,
  Storage.timezone.
- API is only compatible with the multi adapter for now.
- Test with ./remind.py -i ~/.reminders | ./remind.py

Abook:
- Storage adapter for the abook file format, see
  http://abook.sourceforge.net/.
- Needs python-vobject and python-configobj libraries.
- Config file option: storage.abook_file.
- API is only compatible with the multi adapter for now.

Multi:
- Adapter to use Remind and Abook as storage backends at the same time.
- Config file option: storage.type = multi.
Revert "revert of 00674fe"

This reverts commit a94984b6531753d5a48d7838a00e3a699f4756b0.
Add workaround for Bug in Davdroid