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.
Shift learn tabs slightly to the right.
Merge branch 'staging' into test