Configurations

Add the ability to configure how the gem names the '_attributes', '_namespaces' and '_text' keys.

Todos:

- [x] Add XML2JSON.configure do |config| .... end
- [x] Add attributes key
- [x] Add namespaces key
- [x] Add text key
- [x] Specify default values
- [x] Add a method to reset the configuration values
- [x] Use it in the gem!