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
Backwards-compatible for people who are including `cldr.js`, but not `i18n-plurals.js`.