Support for boolean data type conversion.

```ruby
require 'sax-machine'

class Fake
  include SAXMachine
  element :name
  element :main, class: :boolean
  element :published, class: :boolean
end

xml = '<fake><name>Prodis</name><main>true</main><published>false</published></fake>'

fake = Fake.parse(xml)
fake.name      # => "Prodis"
fake.main      # => true 
fake.published # => false 
```