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 
might be beneficial to utilize the ldaptive DN parsing support here:

http://vt-middleware.googlecode.com/svn/ldaptive/javadoc/bundle-1.0.5/index.html

that way it should correctly handle unescaping the DN if needed