Adding the new JsonToStringStyle to ToStringStyle

Implementation of `JsonToStringStyle` inside `ToStringStyle` that outputs the String representation of the object in json format.

Jira ticket https://issues.apache.org/jira/browse/LANG-1015
@britter there is a constructor 
```
public ToStringBuilder append(final String fieldName, final Object obj, final boolean fullDetail)
``` 
that if the `fullDetail` is false the others ToStringStyle ommit the value and shows only the info about the object just like I've implemented. I've followed the way the others are implemented..