Do unused/undefined variable checks in JSHint

 * set the `undef` and `unused` flags
 * define `CLDR, `Ember`, `Handlebars`, and `jQuery` as globals
 * define test framework functions as globals
 * change uses of `Em` to `Ember`
Move CLDR plurals.js into lib/

In Ember-I18n 2.0, the CLDR `plurals.js` library will come built-in.
Now that there are multiple files in `lib/`, we need a (tiny) build
system and distribution file. `make dist` will compile
`dist/ember-i18n.js` and run JSHint on the result.
Always use Ember.Handlebars.get

Now that Ember-I18n only supports Ember 1.0, we don't need to fall back
to using `getPath` when `Ember.Handlebars.get` is unavailable.`
Remove checks for CLDR

Now that Ember-I18n bundles in CLDR's plurals.js, we can be sure it's
available.
Always use Ember.assert

Now that Ember-I18n only supports Ember 1.0+, we can always use
`Ember.assert` and don't have to fall back on `window.ember_assert`.