Add handlebars extractor

First step to issue #11

Missing tests and doc but working nice.

The gettext should be registered in Handlebars as an helper, and all derivate.
Suppose you are using `i18n`

```javascript
var Handlebars = require('handlebars');
var i18n = require('./i18n'); // your own gettext, maybe build upon Jed
 

Handlebars.registerHelper('i18n', function(context) {
    return i18n(context);
});

Handlebars.registerHelper('i18n.ngettext', function() {
    return i18n.ngettext.apply(i18n, arguments);
});
```

In your template

```html
<div>{{i18n "I'm singular" }}</div>
<div>{{i18n.ngettext "dog" "dogs" count }}</div>
```