New Remind, Abook and multi storage adapters

Remind:
- Storage adapter for the remind calendar software:
  http://www.roaringpenguin.com/products/remind
- Needs python-vobject library.
- No bidirectional sync, only reads entries for now.
- Config file option: storage.remind_file.
- API is only compatible with the multi adapter for now.

Abook:
- Storage adapter for the abook file format, see
  http://abook.sourceforge.net/.
- No bidirectional sync, only reads entries for now.
- Config file option: storage.remind_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.