Internalize pluralization

 * remove dependency on `CLDR.js`
 * add `lib/i18n-plurals.js` with pluralization logic, to be included after `lib/i18n.js`
 * add deprecation warning if `window.CLDR` is defined
 * look for current locale in `Ember.I18n.currentLocale` before falling back on `CLDR.defaultLanguage`
 * Change README to describe new process to add pluralization support

Closes #46

See also #49
There are two ways somebody could use *this version* (which will be Ember-I18n v2.9):

### Backwards-Compatibility Mode

Here, the application was on Ember-I18n v2.x and using CLDR.js:
```html
<script src="/vendor/cldr.js"></script>
<script src="/vendor/ember-i18n.js"></script>
```
In this case, we want `Ember.I18n.pluralForm` (which `Ember.I18n.template` calls) to be `CLDR.pluralForm`.

### Forwards-Compatibility Mode

Here, the application is moving towards Ember-I18n v3.x and will be using the internalized pluralization:
```html
<script src="/vendor/ember-i18n.js"></script>
<script src="/vendor/ember-i18n-plurals.js"></script>
```
In this case, `ember-i18n-plurals.js` "installs" itself by setting `Ember.I18n.pluralForm`.