Improving chosen labels internationalization

This pull request extracts AbstractChosen hardcoded default labels into AbstractChosen static fields.

It will allow to globally define chosen labels, depending, for example, on current locale.
That is to say, it will allow to write this code :

        <html>
          <head>
            <link rel="stylesheet" href="chosen/chosen.css">
          </head>
          <body>
            <select id="test1" style="width: 500px" multiple="true">
                <option value="1">1</option>
                <option value="2">2</option>
            </select>
            <select id="test2" style="width: 500px" multiple="true">
                <option value="1">1</option>
                <option value="2">2</option>
            </select>
            <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.4/jquery.min.js" type="text/javascript"></script>
            <script src="chosen/chosen.jquery.js" type="text/javascript"></script>
            <script type="text/javascript">
                $("#test1").chosen(); // It will display, as before, a multiple select chosen, with default label "Select an option"
                AbstractChosen.default_multiple_text = "Sélectionner plusieurs choix"; // Let's override default text to display it in french starting from here... Note that this modification won't modify #test1's select (even after a liszt:updated event) but will modify _every_ chosen select created starting from here
                $("#test2").chosen(); // It will display a multiple select chosen, with default label "Sélectionner plusieurs choix"
            </script>
          </body>
        </html>


Note that this pull request is better than the 688 one :
- A dedicated feature branch for the pull request (instead of working on 4sh/master)
- No rollbacks in commit history, due to non pertinent line modifications in generated js files
- 2 dissociated commits for minified and non minified js files (since there are lots of non pertinent noise in minified scripts : if you want to generated minified scripts on your side, you can avoid the latest commit)