Split the Karma conf file

karma.conf.base.js is the base config file which includes everything but the
files and preprocessors. These are set by karma.conf.js or
karma.conf.coverage.js which load up the non instrumented bundle and the
instrumented one respectively.
Refactor the file patterns
Refactor test tasks

There's now a `test` task which runs the non instrumented bundle. For coverage,
there's the `test-coverage` bundle.
Update README

Mention both test tasks and talk about Istanbul limitations.
Update bundle path in test runner