Stop referencing the TypeScript preview image on the CoffeeScript features page

This PR changes the `Views\features\coffeescript.cshtml` page to reference `~/images/coffee/preview.png`. I have __not__ included an updated screenshot image yet (it is still a copy of the TypeScript image), however after this PR, it should simply be a matter of making a screenshot and changing the image.

Here is a quick screenshot I made, but it doesn't fit the original image attributes (e.g. pixel size, file size, etc).

![preview](https://f.cloud.github.com/assets/570470/1749606/b6ba5940-6528-11e3-98e7-d04e92c103d6.png)