Fix methods for create_attribute() in Twing, and tweak some tests accordingly

[MichaelAllenWarner]

This PR modifies /lib/Attribute.js so that the create_attribute() function correctly returns an Attribute object with methods in Twing.

It also:

• modifies the corresponding 'should return an Attribute object with methods' test (for both Twig.js and Twing) to use .addClass(["class1", "class2"]) instead of .addClass("class1", "class2") (according to the Drupal docs, an array should be used here for multiple classes);
• changes test.failing(...) to test(...) for the Twing 'should return an Attribute object with methods' test;
• accounts for the now-failing 'should export drupal-attribute as Attribute' tests by changing test(...) to test.failing(...) (perhaps it would be better to remove them altogether if this PR is accepted);
• changes test(...) to test.failing(...) for the Twing 'should return an Attribute object with accessible properties' test, which is now failing (as the Twig.js one already was).

The last item (property-access now failing on Attribute objects in Twing) is clearly undesirable, and off-hand I'm not sure how to fix it, but in my experience the methods are far more commonly used than property access, so this strikes me as a good trade-off. Property-access can be tackled later separately.

Fixes #51

[MichaelAllenWarner]

Closing in favor of #54 (which includes this work)