Add option to set naming strategy per object

[tarjei]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	no
RFC?	no

Steps required to reproduce the problem

Given a class like

class Product {
  public $onlyForDevs;
}

1. Add @NamingStrategy("camelCase") to the class
2. Serialize.
3. Get:

{
"onlyForDevs": 1
}

Expected Result

{
"onlyForDevs": 1
}

Actual Result

{
"only_for_devs": 1
}

The point here is to be able to do a partial conversion to camelCase without having to change 1000s of attributes or do a big bang conversion of the complete API.