first go at adding emmet (xml, html and css)

enable emmet style to run on ```xml``` and ```html``` files (closes wolfgangmm/eXide#37)

Additional changes required : 
#### in $shared/resources/scripts/ace/demo.js:
```js
var onChangeMode = function(e, target) {
    var editor = target;
    if (!editor)
        return;
    var modeId = editor.session.$modeId;
    //var enabled = modeId && /css|less|sass|html|php/.test(modeId);
    var enabled = modeId && /xml|css|less|sass|html|php/.test(modeId);
    if (e.enableEmmet === false)
        enabled = false;
    if (enabled)
        editor.keyBinding.addKeyboardHandler(exports.commands);
    else
        editor.keyBinding.removeKeyboardHandler(exports.commands);
};
```

####  add emmet.js in $shared/resources/scripts/ace: 
https://rawgithub.com/nightwing/emmet-core/master/emmet.js

#### Todo : 
might be a good idea to add a preference option to ebable/disable emmet (e.g. switch to previous behavior with xml and html snippet)

Cheers, 
C.