Add support for different resize methods

[tomatolicious]

This adds support for the three (or actually just two, as the default has always been scale) new resize modes in the tinypng API:

• scale: Scales the image down proportionally. You must provide either a target width or a target height, but not both. The scaled image will have exactly the provided width or height.
• fit: Scales the image down proportionally so that it fits within the given dimensions. You must provide both a width and a height. The scaled image will not exceed either of these dimensions.
• cover: Scales the image proportionally and crops it if necessary so that the result has exactly the given dimensions. You must provide both a width and a height. Which parts of the image are cropped away is determined automatically. An intelligent algorithm determines the most important areas and leaves these intact.