Adds Entity DSL

[mbleigh]

This is the simplest implementation I could think of. It works like this:

class User
  include Grape::Entity::DSL

  # simple attributes can be defined inline
  entity :name, :email do
    expose :optional, if: :condition
  end
end

What do you think?

[travisbot]

This pull request fails (merged 7166363 into 3641744).

[mbleigh]

Looks like there's some problem with non 1.9 rubies and this code. Any clues?

[travisbot]

This pull request passes (merged 1b9217d into 3641744).

[dblock]

Very nice, merging.

[dblock]

Add a line to the CHANGELOG please, too.