Upgrade to v2 of KISSmetrics script

Since 2012, the KISSmetrics JS script was updated to v2 (as indicated in
the URL -> scripts.kissmetrics.com/<api_key>.2.js). This updates the
script code that's injected with km.js.

Other changes:

• Extract KissmetricsAPI into separate file
• Make :kissmetrics_api_key and :output_strategy class attr_accessors
  rather than a methods you need to override
• Update ruby syntax to > 1.9
• Update rspec syntax to 3