Implement format_number() for international number formating

I found this when reviewing xmlsitemap module that could make use of this function. The discussion is in #470778: Constant floating numbers shown in the user interface must be translatable. The core API documentation at http://api.drupal.org/api/group/format/6 tells us there are functions to format numbers, strings, dates, etc., but this is partly wrong. There is currently no function that formats numbers with international rules.

In German and many other countries in the EU floating point numbers do not have a dot (0.3). They use a comma (0,3). We should implement this for i18n.