Return keywords as array instead of string

[pdehaan]

Fixes #48

[coveralls]

Coverage decreased (-1.8%) to 98.182% when pulling b88045b on pdehaan:issue-48 into 7e8b27f on mozilla:master.

[pdehaan]

If we want to do something like this, my solution is probably a bad approach and we should use this jazzy new processors[] thing to convert the comma-separated keywords string to an Array.

[pdehaan]

Closing in favor of the processors approach in #61