adds snowplow event plugin

	new file:   vendor/assets/javascripts/plinth/plugins/tbg-snowplow.js.coffee
documents removeData function

	modified:   vendor/assets/javascripts/plinth/plugins/tbg-snowplow.js.coffee
doc fix

	modified:   vendor/assets/javascripts/plinth/plugins/tbg-snowplow.js.coffee
Fixing naming error from GA to SP
Cherry picking SHA#95623c58c965f1adc326d73d75d0f239a6259980
Partial cherry-pick of commit SHA#d5399eada9c9c76a1e9c8be2d99645cf1cec3c32 which fixes the explode issue for snowplow event data
Merge pull request #74 from thebeansgroup/dok/feature/snowplow

Adding current version of the snoplow event to the DOK version of plinth/styleguide