Use #to_h instead of #as_hash for retreiving the original hash

In response to #2:

Ruby 2.x+ has been using `#to_h` to denote hash conversion.

See this blog post for more information: [All About #to_h in Ruby 2.x](http://www.benjaminoakes.com/2013/03/08/all-about-to_h-in-ruby-2/).

- [x] Rename `Dish::Plate#as_hash` to `Dish::Plate#to_h`
- [x] Create an alias to the original `#as_hash` method
- [x] Define `Dish::Plate#to_json`
- [x] Deprecate `#as_hash` (run `#to_h` in addition to printing a message)