karma: move Travis configuration into config file

This let's us shorten the command lines in the Travis config file and
better reuse/edit the configration later.
karma: run without coverage by default

Coverage generation slows down what is supposed to be very fast
continiously running tests.
npm: add coverage script

This runs Karma and continiously generates coverage data.
travis: send coverage data to coveralls.io

This is a free service that will host the generated coverage reports.
It will also comment on open pull requests with info about the change
in coverage caused by that pull request.