Provide UserManager addToGroup() and removeFromGroup()

@frett review please

This PR provides UserManager and UserDao API's for adding and removing users from a group.

Note that these changes will require that all projects defining spring bean `LdaptiveUserDao` and/or `UserLdapEntryMapper` be updated. @dsgoers 
It does, and it leaves the values unescaped when it concatenates the values back together. Either the way it is now or the way it was before won't always do the right thing if there are escaped characters in RDN values. We will just need to make sure to avoid those special characters.

I was hoping more for something along the lines of:

```
List components = DnParser.convertDnToAttributes();
for(Attr component : components) {
    path[] = component.value;
}
```
with appropriate error checking if the attributes were of the wrong type. That way the DN is split before we unescape, and then the values are unescaped according to the standard and made available in their component form.

We could use a couple calls to DnParser.getValues to get the path components and the name, but that would ignore any other types of RDNs in the middle of the path that we may want to throw an exception on.