Conventional changelog

This commit adds the grunt-conventional-changelog task to restangular's package.json. You're now able to automate your changelog generation by doing

````
$ grunt changelog:from_ref:to_ref
````

`to_ref` is `HEAD` by default. So if you want to generate a changelog from 0.5.5, update the version number in your package.json (since this is currently the version number which is take by grunt-conventional-changelog) and run:

````
$ grunt changelog:0.5.5
````

The Angular project uses these conventions and many others too! More information: https://github.com/btford/grunt-conventional-changelog