Replace the support of <code> elements by <script>

The <script> element is better than <code> element, because:

- no need to remove the element to avoid side effects
- this element is ignored by the browser, so no execution, no rendering, no re-layout, no pollution of the view (when uilang.js is not executed because of an exception made by an other script)
- This element is ignored by search engines
-  we can have many <script type="text/uilang"> elements in the page: this is useful when a page is built from several files that have their own uilang instructions
- It respects the HTML semantic: <code> is to display code, not to execute code
- uilang can be extended by supporting external uilang files with `<script type="text/uilang" src="file.uilang"/> (having such features with <code> is ugly...)