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

this adds brian's awesome grunt task to generate changelogs automatically
through well-formed commit messages.

there's now a new section in the gruntFile.js called 'changelog', which
specifies the options for the task.

for more information about the grunt plugin checkout btford/grunt-conventional-changelog
its readme file also describes where you can find the specification for
well-formed commit messages.

when running `grunt changelog:FROM_REF` make sure `FROM_REF`  is an existing tag in your repository.