Add support for deprecated methods

Adds support for deprecated methods.

A deprecated method behaves exactly like a normal method, the only difference is that it is not returned in the list returned by `available_methods` class method.

An example:

```rb
class CircusPresenter < Curly::Presenter
  attr_reader :midget, :clown, :elephant, :horse

  deprecate_methods :horse
end

...

circus_presenter.available_methods = [:midget, :clown, :elephant]
```