Add support for things needed by Webmaker (plist, URL-based locales). Closes #38

This makes a number of additions, without changing the default ways in which abide previously worked.  They include:

* `translation_type` option, which allows `plist` files to be supported.  The default remains `json`.
* Support for `plist` files in addition to `json`
* `locale_on_url` option, which adds optional URL and header munging based on the presence of langague tag information on a URL.  For example http://foo.com/fr/index.html means use the French language locale for http://foo.com/index.html.
* Addition of a dynamic getter `getLocales` to discover at runtime the list of locales currently supported

In a subsequent pull request/bug I'd like to do more elaborate tests, add support for automated tests in Travis, etc. but didn't want to grow this patch beyond what was necessary.