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 loop and the subsequent loop can be combined, there is no need create an internal LdapEntry object to hold attributes that you are immediately extracting for the modifications List.