Don't try to save entry if there were no modifications

This seems to cause problems with the unique overlay.

Fixes #28
Fixes #33
Make hash function in LDAPAttribute dependent only on attribute name

This ensures that the hash of the object stays constant for the lifetime
of the object, which is required for hashable objects. The hash would
previously change when the objects in the set changed, resulting e.g. in
membership tests for sets failing.
Bump version to 0.12.1