Conversion to ember cli addon

This should eventually resolve #11

# Latest

The current commit of this pull request makes `ember-gdrive` run as an `ember-cli` addon and the Storypad app is able to run with it instead of the default library with no obvious errors.

~~I was to quick on the trigger there. store extensions aren't being properly loaded, so store operations throw errors. I'll try to find the best way to include the script as part of the addon.~~

**Moving the `store-extensions` initializer to the `app` folder did the trick.**

I believe most of the work is done, but there's still some cleanup and restructuring required. 

While doing the conversion, I also got rid of most of the `jshint` warnings by adding proper imports, formatting, etc. A few remain, but not many.

## Remaining tasks/questions:

* [x] ~~There's a certain folder structure an `ember-cli` addon should conventionally follow. If we do follow it, then some of the `import` statements in any app using it need to be updated. If we do not follow it, then there's no need to update the links. I already went with the "follow the proper structure" direction, but I could revert if needed.~~ **We ended up agreeing on following conventions, even if it means doing some "import reference updating" in Storypad.**
* [X] ~~The addon extends/reopens/injects some stuff into the main app. For now, I mostly went with initializers inside the `app` folder of the addon, so they become part of the main app and are loaded automatically. I'm pretty sure that's the conventional way and it does work, but I'm not completely sure yet.~~ **Initializers ended up being the way to go.**
* [X] ~~I didn't get a chance to check, but it's possible `addon/lib/library.js` doesn't get loaded at all currently. I should probably transform that into some sort of initializer as well, or possibly even move it to the `index.js` file.~~ We decided there was actually no need for the code inside the `library.js` file.
* [X] ~~`gapi` and `google` objects used in some of the modules should probably somehow be accessible as modules as well.~~ **As agreed, we will keep them global for now, but we can revisit it in the future.

## Tasks that could go to separate issues/pulls once this is merged

Once everything is ready, I could deal with the `ENV is undefined` issue as well as the load order issue (in relation to `ember-simple-auth`). I think those two should be separate issues/pull requests.

* Fix `ENV is undefined` issue by accessing the app configuration (made accessible by the conversion) and retrieving the `GOOGLE_CLIENT_ID` that way.
* Fix load order issue by implementing a configuration property that can be set through the main app, which would provide a list of addons `ember-gdrive` needs to be loaded before.
* Move authentication from storypad into the addon. How far should we go?

# Previous

~~This is just an initial commit for now. I was doing some research so I started to do the basics.~~

~~I also pulled the changes from the "quick and dirty fix" for load order, so I can see if I can do something about it while handling the conversion. I'll probably have to rely on some sort of configurable property to make sure things are loaded before `ember-simple-auth`.~~