Support for Symbol data type conversion.

```ruby
require 'sax-machine'

class Fake
  include SAXMachine
  element :name
  element :status, class: Symbol
end

xml = '<fake><name>Prodis</name><status>sleepy</status></fake>'

fake = Fake.parse(xml)
fake.name   # => "Prodis"
fake.status # => :sleepy 
```