Added generation of combined report

At the end of execution SalsaRunner generates also overall report, which is covering results of multiple browsers (or Browserstack combinations).

Please note that most of changes in salsa_webqa/library/report/resources/style.css were caused by WebStorm auto-formatter - no real changes, except from lines 25-36, have been made to the file.

Requires new python package: "jinja2" (_pip install jinja2_).