Adds OAuth authentication for the new API
Updates the client with the OAuth authentication system
Serializes requests as JSON because the new Ecwid API doesn't
accept form encoded values anymore.
Refactors each of the API classes into an `Api` module.

Also, adds the ability to create and update products.
Implements the `#camel_case` method on `String` only if it isn't
already there.
Updates the README so it reflects API v3
Changes the Authentication class to OAuth

Authentication was a poor name for the class. This will also
add a few specs for OAuth as well as an entry in the README.