Upgraded /images in Pegasus to better support Retina assets.

Moving forward, ideally the *only* images that would be checked in would be the retina `@2x` versions, e.g. `/images/bearclaw@2x.png`. Requesting `/images/bearclaw.png` will resolve the `@2x` version and reduce it to half the original size when delivered. If both `bearclaw@2x.png` and `bearclaw.png` files exist, the right file is selected an returned unmodified.

Now, **all** images support automatic type translation, e.g. when `/images/bearclaw.jpg` is requested Pegasus will locate `bearclaw.png` (or `bearclaw@2x.png`...) and translate the image appropriately.

The manipulation features (`fit`, `fill`, `resize`) now support height-only specifications in addition to width-height, and width-only ones. For example, `/images/fit-x100/bearclaw.jpg` will constrain only the height when resizing.

PNG is the preferred format for images when committed, but JPG is almost always the preferred format when referencing on the page. A 300 KB PNG is generally <60KB as a JPG reducing our bandwidth costs and improving site performance. The most frequent exception is when an image contains transparency that needs to be preserved.