Bugfix/gh782 acl xml format

The root cause for #782 was misuse of `xmerl_scan:string/2`, which doesn't emit spaces, comments between actual xml elements. It leaves them as `#xmlText{}` and `#xmlComment{}` - This pull request commit changes to ignore them at walking around parsed dom tree.
This diffset also includes a small refactoring that wraps all `xmerl_scan:string/2` to one `riak_cs_xml` funcsion to control its usage.