Added support to guess numbers.

I've added the possibility to generate a more typed JSON by requiring to guess if a value seems to be a number.

I'm aware that this is a very trivial solution and can be considered a POC. 

It would be nice that future versions will provide the mechanism to generate a typed rich JSON that could be easily consumed by a strong typed JSON API.

In any case I like the overall flexibility in managing the mapping form HTML input fields to JSON hierarchy.

Another nice feature would be the possibility to selectively map real _null_ values.

Regards