Modules can require external libs

adding #requiresLib function to modules
use it like that:

```js
module('foo')
.requires('bar')
.requiresLib({
    url: 'http://some/lib.js'
    loadTest: function() { return typeof someLib !== 'undefined'; }
})
.toRun(function() {
  // code that depends on module('bar')
  // and http://some/lib.js
})
```

See apps.Handlebars for real-world usage.