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 
This unit test has no need to be configured in spring, it's testing the functionality of the GroupDnResolver, which is fairly simple and could be statically configured for this test.