Adding handler support for HTML entities.

Currently, markdown2pod crashes if it encounters an HTML entity such as `&amp;` or `&lt;` in the source text. This is because its handler does not define the `html_entity` method that Markdent::Role::EventsAsMethods expects.

This patch provides this method, transforming the given HTML entity into a POD `E<...>` entity. It also provides a trivial test for it.