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 
You don't handle InvalidGroupDnException here. It might be worth converting InvalidGroupDnException to a checked Exception so that it doesn't get missed in code that utilizes .resolve()

My opinion is that groups that don't parse correctly should be ignored with maybe a debug log message.