Coverage-Assets are not referenced correctly when writing to a sub-folder

[pesse]

Finding by @tkleiber from #48

The new folder raises a problem in combination using directories in the -o switch eg.:
-o=results/ut_coverage_html_reporter.html

The resulting ut_coverage_html_reporter.html contains too the reference to this directory whis is wrong:
results/ut_coverage_html_reporter.html_assets/application.js

This should furthermore be:
ut_coverage_html_reporter.html_assets/application.js
as it must be relative to to html file location.

As workaround I change now into the directory before calling the cli and use -o without directories.