compileLessToCss transform: Switch from seq to async.
compileLessToCss: Only try to load the less compiler if less assets are actually found.
compileCoffeeScriptToJavaScript: Only try to load the less compiler if coffeescript assets are actually found.
compileStylusToCss transform: Switch from seq to async.
compileStylusToJavaScript: Only try to load the stylus compiler if stylus assets are actually found.