Change the way itemids are assigned to carddav contacts

itemids used to include the full folder path, and a filename
based on a md5 hash at time of creation. The folder path creates
a problem when used as an itemid on another carddav server.

This fixes: https://github.com/skarra/ASynK/issues/41
Check principal is found during init.
Add a new test method to clear a folder of all contents from the server
Simple Makefile for the test directory
Improve notifications during sync.