Removing Handlebars dependency

This PR replace Handlebars dependency by simple templating function. The code isn't getting any bigger but it may loose some of it's previous flexibility provided by `Handlebars.compile`. For now the test suite is passing, but there may be some edge cases. Please consider if it is worth to go this way.
I'm quite surprised that didn't work with the Handlebars version. Maybe I'm forgetting my plain Handlebars after working with Ember.Handlebars so long...

Here's a case we probably *should* support, though it does seem to be a new feature:
```js
Em.I18n.translations['some.key'] = 'Some Key: {{myProperty}}';

var context = {
  unknownProperty: function(property) {
    return property + ' value';
  }
};

Ember.I18n.t('some.key', context); // 'Some Key: myProperty value';
```