Reapply "Add initial support for the line-height property. Line height is only based on the tallest box in each line and does not factor in the vertical-alignment. Improves whitespace handling by passing the whitespace state between function invocations."

This reverts commit 0bb3fbdde0b59a5d3df38a7fd7b970633300585c.
Fix line height for "normal" line-height styles.
Fix computing line height for generic boxes and images.
auto merge of #532 : jrfeenst/servo/line-height3, r=pcwalton

Wikipedia, lipsum, and everything else I looked at seems fine now.