Add Lorem Pixel Support w/ Categories

Adds [LoremPixel](http://lorempixel.com) placeholders as of 11.08.2014
* Used like so `define( 'MISSING_UPLOADED_IMAGE_PLACEHOLDER_BUILTIN', 'lorem_grayscale' );`
* Or you can use a slug of say sports: `define( 'MISSING_UPLOADED_IMAGE_PLACEHOLDER_BUILTIN', 'lorem_sports' );`
* The following slugs are supported with this version: abstract, animals, business, cats, city, food, nightlife, fashion, people, nature, sports, technics, transport