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 
`DnParser.substring(...)` calls `DnParser.convertDnToAttributes(...)` http://grepcode.com/file/repo1.maven.org/maven2/org.ldaptive/ldaptive/1.0.3/org/ldaptive/DnParser.java