fix for anonymous block boxes
remove block exception
remove unused import
add comment
fix comment
auto merge of #1461 : ksh8281/servo/block_black_boxes, r=SimonSapin

fix for anonymous boxes.
http://www.w3.org/TR/CSS2/visuren.html#anonymous-block-level
fix try_to_append_line for ImageBox
auto merge of #1463 : ksh8281/servo/img_new_line, r=pcwalton

````
<div>
   test
   <img src="test.jpeg"/>
</div>
````

if reduce window size, img is disapper.
this pr fixes this case.
Implement Document::head()

Implementation details according to the specification below:
http://www.whatwg.org/specs/web-apps/current-work/#dom-document-head

This patch is for:
https://github.com/mozilla/servo/issues/1465
auto merge of #1466 : brunoabinader/servo/document-head, r=kmcallister

Implementation details according to the specification below:
http://www.whatwg.org/specs/web-apps/current-work/#dom-document-head

This patch is for:
https://github.com/mozilla/servo/issues/1465

Closes #1465.