Refactor diff hierarchy into sections
Refactor diff hierarchy into sections

Added a Section class that handles the common logic of finding header
and content lines and creating subsections. It includes all the logic
from the Parser class, which was removed, plus it includes the logic
from File#get_header_lines, which was also removed.

Made Diff, File, and Hunk subclasses of Section. APIs remain almost the
same. All classes now have #text and #subsections. Hunk no longer has
#lines.

wylee/review