Add `breaking changes` section, `github` option.

I added breaking changes section, similar to angularjs's changelog.

I also changed the way to find the previous version from a for loop to a search for the previous tag - again, similar to angularjs's way.

I also added a `github` option, which will make commits link to a github repository, if a github repo is found.  If user does not specify `github` option, it will try to find the github repository from `repository` or `homepage` fields of `grunt.config('pkg')`.