Mix & Extract single JSON property

Salut Benoît, Yves,

I've committed some functionality developed by me and Max. These features are:

- mix(String... fields) - mixing two or more properties or sub-properties  
- extractOnly(String field) - extracting single field

I've run cobertura coverage and everything that we've added is covered. This holds true for the changes we've made as well.

Let me know what you think and if you accept these features I will write a new blog post.

Cheers,
Ivan