Implement Document's 'body' attribute

Implement Document's 'body' attribute (getter and setter). Proper
implementation of setter requires Node::ReplaceChild(), which is
currently a stub and will be done on a later step.

This patch is for:
https://github.com/mozilla/servo/issues/1428
auto merge of #1431 : brunoabinader/servo/document-body, r=jdm

Implement Document's 'body' attribute (getter and setter). Proper
implementation of setter requires Node::ReplaceChild(), which is
currently a stub and will be done on a later step.

This patch is for:
https://github.com/mozilla/servo/issues/1428