possible to load amber.js by RequireJS itself

In addition to inline `<script>` tag, it is now possible
to load amber.js (that does all the RequireJS mappings)
by RequireJS itself (mapping just 'amber' and loading it).
So, instead of:

```
<!DOCTYPE html>
<html>

<head>
    <title>Amber Smalltalk</title>
    <meta http-equiv="content-type" content="text/html; charset=utf-8"/>
    <meta name="author" content="Nicolas Petton"/>
    <script type='text/javascript' src='support/amber.js'></script>
    <script type='text/javascript' src='support/requirejs/require.min.js'></script>
</head>

<body>
<script type='text/javascript'>
    require(["amber/devel"], function (smalltalk) {
        smalltalk.initialize({'transport.defaultAmdNamespace': "amber_core"});

        smalltalk.globals.Browser._open();
    });
</script>
</body>
</html>
```

one can put:

```
<!DOCTYPE html>
<html>

<head>
    <title>Amber Smalltalk</title>
    <meta http-equiv="content-type" content="text/html; charset=utf-8"/>
    <meta name="author" content="Nicolas Petton"/>
    <script type='text/javascript' src='support/requirejs/require.min.js'></script>
</head>

<body>
<script type='text/javascript'>
    require.config({paths: {amber: 'support'}});
    require(["amber/amber"], function () {
        require(["amber/devel"], function (smalltalk) {
            smalltalk.initialize({'transport.defaultAmdNamespace': "amber_core"});

            smalltalk.globals.Browser._open();
        });
    });
</script>
</body>
</html>
```

The mapping must be loaded first, then subsequent parts later
in the nested require call.
Merge pull request #905 from herby/load-amberjs-by-requirejs

possible to load amber.js by RequireJS itself