Support for +∞ and -100% in change columns

This fixes #170.
Fix 0/0=0 in change column
Implement “latest significant results” table and feed

This fixes #171
Refactor feed generation a bit
Fill the RSS feeds with a bit more content

Especially the summary is useful
Include the full changes table in the feed

(by putting it into a template of its own)
New color "light green" for positive trends

to distinguish them from non-trend improvements.
Simplify classification logic for the summaries

(changes results before trends, averages before individual results)
Integrate code review
Merge pull request #172 from nomeata/submit

Two more changes