Adds render callback option for init

This is the Retina helper wrapper module. 

Example setup in `app/lib/setup.coffee`:
![1 shell](https://f.cloud.github.com/assets/356868/856566/71a65c18-f53f-11e2-9c1f-22a2fcbbe798.png)

Then, we can call it by:
```coffee
MemberApp.Enhance.render(@member.profile_photo_url, width: 50, height: 50)
```

And based on whether or not the device has a high DPI, it will double our dimensions we request from Monocle.

There's some default behavior in there as well, such as simply suffixing images with the Apple convention `@2x`. I'd like to make this more generic rather than tailored to Crowdtap's needs.