Fixed require.js support

List of changes:
 * Unified require.js support blocks on three artifacts (beautify, beautify-css & beautify-html) using the standard `define` usage suggested at https://github.com/jrburke/r.js/issues/558#issuecomment-33546625
 * Changed dependency definitions in `beautify-html.js` file to avoid using relative paths *and* extensions, as it generates a problem on second degree dependency loading.