Un-break Karma, replacing manual index.html

The existing config assumed we would name our tests _spec.js, but we stopped doing that.

Karma gives us a number of benefits:

- Not having to maintain index.html or all.js manually
- Ability to run entirely from the command line
- An easy command-line watch mode
Add a CONTRIBUTING.md explaining how to run the tests
Merge pull request #28 from dglazkov/karma

Move back to Karma as our test runner