feat(conventional-changelog): adds grunt-conventional-changelog

this commit adds the grunt-conventional-changelog plugin.
with this plugin it is possible to auto generate changelogs based on
commit messages, as long as they follow a specific convention.
these conventions are also used by angular.js itself. :)
Merge pull request #3 from PascalPrecht/grunt-conventional-changelog

feat(conventional-changelog): adds grunt-conventional-changelog