Changing the yielding API

[rolftimmermans]

This PR changes the API of Jbuilder to not require an argument for every block. It also fixes the implementation of cache! to be compatible with this API (see confusion in #63).

Old API:

Jbuilder.encode do |json|
  json.author do |json|
    json.name "Rolf"
  end
  json.attachments @message.attachments do |json, attachment|
    json.filename attachment.filename
  end
end

New API:

Jbuilder.encode do |json|
  json.author do
    json.name "Rolf"
  end
  json.attachments @message.attachments do |attachment|
    json.filename attachment.filename
  end
end

Documentation has been updated, but the old API actually still works. Keeping backwards compatibility was only an issue for the few methods that take a block with 2 arguments. The first argument used to be an instance of a Jbuilder object, the second argument the element for the current iteration. With this change a block with arity 1 only receives the element. The code for that is ugly though. Therefore it would be great if the old API could be deprecated and removed sometime after a major version bump.

[dhh]

Love it!