Fixes yaml#25. Rebase and tidy up of yaml#63.

This reverts commit 3a1452c.

The tests will break on python 2.7 under win and macos.
They were renamed in yaml#261

* Fix logic for quoting special characters

* Remove has_ucs4 from condition

on systems with `sys.maxunicode <= 0xffff` the comparison
(u'\U00010000' <= ch < u'\U0010ffff') can't be true anyway I think

* Update CHANGES

Some PRs were forgotten, and some were listed that weren't merged.

* Change default loader for yaml.add_constructor

If the Loader parameter is not given, add constructor to
all three loaders

When someone writes a subclass of the YAMLObject class, the constructors
will now be added to all 3 (non-safe) loaders.

Furthermore, we support the class variable `yaml_loader` being a list,
offering more control of which loaders are affected.

To support safe_load in your custom class you could add this:

    yaml_loader = yaml.SafeLoader

    yaml_loader = yaml.YAMLObject.yaml_loader
    yaml_loader.append(yaml.SafeLoader)