Adds support for Piwik

Adds support for [Piwik](http://piwik.org/), supersedes [GH #119](https://github.com/segmentio/analytics.js/pull/193), and resolves concerns brought up in [GH #19](https://github.com/segmentio/analytics.js/pull/19)

Piwik's Press Pack can be downloaded from http://piwik.org/piwik_press_pack.zip, which contains the vector image that should be used.

Thanks to @mattab for providing a permanent place to run the tests against.

@ianstormtaylor @noinput @halfdan @patcon @sethers @vanthome