Preserve incoming source maps

When I supplied ```transform: ['coffeeify', 'uglifyify']``` to grunt-browserify, the CoffeeScript-to-JavaScript source maps that coffeeify produces got lost, because uglifyify ignored them.  With this patch, uglifyify is now aware when incoming source code contains an existing source map and passes it through to uglify-js, so that errors in the uglified-browsified-compiled JS can map back directly to their CoffeeScript source code.