Implement 'vertical-align' and fix 'line-height'.
Add a test case for vertical_align and line-height
Update linebox's height which is changed by vertical-align.
Fix the line-height error in a multi-line page
Fix and add comments. Use `map_default`.
auto merge of #764 : june0cho/servo/verticalalign, r=metajack

This is a modification of the previous PR #741 to support 'vertical-align' and 'line-height'. Added test cases for vertical-align, line-height, and mixed.

In the last commit, after applying 'vertical-align' property, the current line-box's height and following line-boxes' heights are being updated. This is because line-box's height was already assigned when the line-box is created. 
But there may be a better way, though I'm not sure. In Gecko, line-boxes' heights seem to be assigned at one time after 'vertical-align'. @metajack