chore(CHANGELOG): add CHANGELOG
fix(package): add missing time-grunt dependency
feat(changelog): add grunt-conventional-changelog

will help generate CHANGELOG.md by following angular.js conventions.
[btford/grunt-conventional-changelog](https://github.com/btford/grunt-conventional-changelog)
[convetions](https://github.com/ajoslin/conventional-changelog/blob/master/CONVENTIONS.md)
feat(doc): add a  task to generate doc