feat(build): Add a `grunt version` task to stamp a new version.

* Adds grunt-version and grunt-conventional-changelog dependencies.
* Updates the version number in package.json
* Updates CHANGELOG.md
* Creates a tag.
* Commits everything.

Closes #496
Merge pull request #594 from mozilla/issue-496-changelog

feat(build): Add a `grunt version` task to stamp a new version.