Mirror Node type properties into Node.prototype.  (github #14)

Webkit defines document.ELEMENT_NODE, but domino previously defined
only Node.ELEMENT_NODE.  This patch adds the node types to Node.prototype
so that they are inherited by instances of Node and its subclasses.
Merge pull request #15 from cscott/gh-14

Mirror Node type properties into Node.prototype.  (github #14)