Improve coverage templates
Fix width of percents
Fix coverage treemap template : now using cdnjs.com to deliver scripts
Coverage and treemap URL are not altered by atoum

When setting up coverage and treemap reports, the user has to provide
root URLs for both html reports.

Basically, we use the
`atoum\report\fields\runner\coverage\html::setRootUrl`and
`atoum\report\fields\runner\coverage\treemap::setTreemapUrl` methods
passing them URLs to reports output directories.

When those report are hosted on a web server, we use something like
`$coverageHtmlField->setRootUrl('http://localhost:8888')`. With a
standard web server, this would bring us to the report's index page.

When we do not want to have a webserver running, we can use file URLs:
`$coverageHtmlField->setRootUrl('file://path/to/project/coverage/index.html')`
to directly bring the user to the index page.

Before this patch, atoum would have altered the URLs, adding a trailing
/ which could lead to problems with file URLs.
Merge branch 'pr/324'