Support uniqueMember with DN for user names.

Extend ldap support with the possibility to use ```uniqueMember``` attribute instead of ```memberUid```

Unlike ```memberUid``` which contains raw user names, ```uniqueMember``` contains ```DN``` that has to be parsed to get the raw user name.

Some LDAP schemas seen in the wild don't contain the ```memberUid```.

This patch allows the usage of  ```uniqueMember``` instead of ```memberUid``` to add support of such setups to DAViCal.