Image preview draft

Hi! 

It's not a final pull request, but I wanted to start a discussion about the feature. I've implemented the image preview in Brackets, but I don't like the way it's currently done. That's how it works:

* DocumentManager sends file path if it's an image instead of reading it as text;
* Document has a "template" parameter which is supposed to point to a custom rendering method;
* Editor _substitutes everything inside the CodeMirror wrapper for a custom template_ if the template name is set in the Document.

Now, the emphasised part is something I strongly dislike, but I couldn't find a way around it. Opening an image as a document feels like a natural way of doing this (that's how it's done in Espresso.app, for example); opening a modal for image file types, while a viable alternative, feels inconsistent in terms of user experience and blocks the UI.

As for the current architecture, Brackets is strongly tied to CodeMirror and there is no way of consistently working with document area without using at least the wrapper (which is understandable, Brackets being a text editor and all). It would be perfect to have not just an Editor class, but a Viewer class with custom templates (somewhat like I did it in this fork) as well, and an ability to switch between the two depending on a file type, but I'm not familiar with Brackets well enough to even know if it's needed and supported by the core team, so I did what I did.

Currently, there's a few things I certainly need to do with this request before making it suitable for merging:
* code conventions (really sorry for that one!)
* template events (or at least a "before render" event for the sidebar)
* template files (right now there is a temporary and ugly workaround)
* status bar (something has to be done instead of the hack in the current version, but I'm not sure what)

But before doing all this I need to make sure somebody actually needs this, and maybe somebody from the core team could help out with Editor/Viewer (or at least discuss this in detail) and the status bar.

Ugly architecture aside, it works as intended on both platforms: shows an image preview and dimensions in the status bar. It could be a start, I guess.

Regards,
Ed.

![screen shot 2013-07-17 at 8 07 57 pm](https://f.cloud.github.com/assets/145218/811825/f1caba38-eee1-11e2-8e99-5a7922bd0143.png)