Make CssSelector::Methods play nice with Memoizable/Mutant

Because the Module.included implementation didn't call super, concrete
classes weren't getting Memoizable's module methods like memoized?,
which in turn confused Mutant.
Add Hexp::Node#append as a convenient API for adding child nodes
Make Unparser Adamantium-immutable, skip escaping inside <script> tags

* Make Unparser Adamantium-immutable so instances can be included in
  Adamantiumized objects
* Skip escaping inside <script> tags
Prepare v0.4.0, bump version, update changelog