Added ability to pass a Proc to the look_like matcher

Example:

  require 'time'

  let(:valid_datetime) { ->(v) { Time.parse(v) } }
  let(:expected_response) do
    {
      :status => 200,
      :body_exact => { 'created_at' => valid_datetime }
    }
  end

  response.should look_like expected_response
Merge branch 'hh/proc_matcher'