Use Karma to run the tests and determine test coverage

Add a Karma configuration and adapter.
With Karma it's possible to run the tests unattended in real browsers and on real devices.
This way, our continuous integration can be more thorough.

  To run these tests, install karma with
      npm install karma-coverage karma-junit-reporter
  Then run 'node ../node_modules/.bin/karma start tools/karma.conf.js' from
  the directory 'webodf' that contains the 'tools' directory.
  This results in a folder called 'coverage' with the coverage information of
  the tests.