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
I guess I don't understand, then - `CLDR.pluralForm` wants to be assigned to `Ember.pluralForm`? Is anything calling `Ember.pluralForm`? I was thinking the point of this line was to set `Ember.I18n.pluralForm` to reference the CLDR version if it was defined.