search keys path in global context if no match found

Details of the changes:

1. Try to get from [globals](https://github.com/linkedin/dustjs/wiki/Dust-Tutorial#setting-your-own-context-before-render) if can not find the ordered keys path in the context stacks.
Similar to built-in version of `context.get()` https://github.com/linkedin/dustjs/blob/master/lib/dust.js#L339

2. Add more unit tests.

3. To ensure all existing and new unit tests are passing, add temp fix (i.e. change 'medium' to 'long' format for date) for the existing broken test. The existing broken test is due to the changes in locale-data's date in https://github.com/andyearnshaw/Intl.js/blob/master/locale-data/json which [does not define 'short' month in its list of combinations](https://github.com/andyearnshaw/Intl.js/issues/65#issuecomment-43190875). The best match algo will return `"month": "long", "day": "numeric", "year": "numeric"` instead.