XSDDocument.make() and XSLDocument.make()

This version fixed a few bugs and introduced two new static methods: XSDDocument.make() and XSLDocument.make(). They are useful when you need to create a reusable artifact, for example:

class Foo {
  private static final XSD SCHEMA = XSDDocument.make(
    Foo.class.getResourceAsStream("schema.xsd")
  );
}

This make() method re-throws IOException as a RuntimeException.