Added support for assigning a class to an attribute

[swunderlich]

Support for the classes Integer, Float, Time on attributes

class ExampleRow
  include SAXMachine
  attribute :created_at, class: Time
  attribute :counter, class: Integer
  attribute :exchange_rate, class: Float
end

I don't think other classes make that much sense. If none is given it still returns a string. I think the pending test can be removed as it is not a common case. (And I can't get my head around it right now :) )

[krasnoukhov]

Thanks for the PR. I'm going to make some changes and cut a release shotly