chore(grunt): adds grunt-conventional-changelog

this commits adds the grunt-conventional-changelog plugin to auto
generate changelogs over time

the conventions are used by the angular.js project as well as
other well known angular modules out there

conventions: https://docs.google.com/document/d/1QrDFcIiPjSLDn3EL15IJygNPiHORgU1_OOAqWjiDU5Y/edit

example output: https://github.com/karma-runner/karma/blob/master/CHANGELOG.md