refactor: use onExit instaed of explicit emitter
feat: add a default config and normalize outputFile path

We used to do this in Karma's config, but it really belongs here.
feat: log where the coverage reports are generated
test: fix the unit tests
fix: update to work with Karma 0.11

Since Karma executes immediately, we don't have all the browsers in advance, the reporter has to listen on browser_start instead.

This re-introduces b744d6f2d01eb37b7160f5e82885e597132a167a but in a back-compatible way.
feat: support coverage for coffee script