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.
Can you please provide an example? I cannot get the case that works with handlebars compiler but not with this one. Have tried in this way (having a translation `'foo.bar.named.long': 'A Foobar named {{name.first}}',`):
```
it('interpolates long keys', function() {
    expect(Ember.I18n.t('foo.bar.named.long', {
      'name.first': 'Sue'
    })).to.equal('A Foobar named Sue');
  });
```
but it worked with new compiler while it is not with Handlebars.