Improved typecasting and XML namespace prefix detection.

Hi @rubiii,

I currently have to implement a SOAP web service for rails (using *wash_out*).
I tested the services using savon and my specs keep failing because nori's
typecasting doesn't work properly (namespace prefix in nori is hard-coded).

So I decided to refactor nori. So far I've added:

* improved type casting
* optional XML namespace detection

Pls have a look at the [updated README](https://github.com/Drachenfels-GmbH/nori#typecasting)

What do you think about it? 
Can you include it in the next release of nori?

Cheers,
Ruben