Implement HTMLDocument properties

Implement missing properties such as:

• $document->title
• $document->images
• $document->anchors

PHPDocs on all classes

All DOM interfaces that are currently within the repo are marked up with their PHPDoc blocks.

Certain methods have been moved to their correct locations and tests adjusted accordingly.

Basic DOM implementation

The basics of the modern DOM implementation we have been using within browsers.

There are a few known gaps in functionality, and tests need to be hardened before a production release.