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 
for now let's revert it back to what you had, I'm going to submit a pull request to make convertDnToAttributes public and down the line we can update this code then.