Rename `Dish::Plate#as_hash` to `Dish::Plate#to_h`

+ Create an alias to the original `#as_hash` method
Update dish_test.rb to use `#to_h`
Define `Dish::Plate#to_json`
Add RubyMotion support for #to_json

+ Check if JSON has been defined
Merge branch 'master' of https://github.com/lassebunk/dish into to_h
Update plate.rb
Add a deprecation notice to Dish::Plate#as_hash
Add notes about #to_h and #to_json to README.md
Test setting values in test/dish_test.rb
Revert test/dish_test.rb

I'm obviously too tired to be swapping contexts in two PRs...
Merge branch 'pr/6'