Add code coverage to CI

This switches coverage from jscoverage to blanket (as suggested a while back by @indexzero)
and configures the Travis-CI build to generate a coverage report and send it to coveralls.io.

You can run:
  make cov: to generate an HTML coverage report (for local viewing)
  make travis: to run the full CI build (lint, test, coverage) and report to coveralls.io

Note that if you run make travis locally it will look for a .coveralls.yml file, so you can
test it by reporting coverage to your fork.