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's not required as you say, but I think I prefer it simply because the bean is already defined and the overhead to have spring build and inject is minimal. In addition, and maybe more particularly, I prefer this since the bean relies on a property setting which I would have to duplicate in the code (or read it myself from the properties).