ISPN-3340 Switching from Json.org to Jackson

Hi

Please take a look at proposed solution for https://issues.jboss.org/browse/ISPN-3340.

The main idea is to create JsonObject which is the only part connected to particular JSON implementation (in this case Jackson). This way switching to another implementation would not require any other changes. 

Additionally I changed Exception policy a bit. All methods which write JSON from objects/maps should not throw any Exceptions. There is only one case where such thing occurs - when there is no fields for serialization. In this case we throw an Exception (there is even test for it). 
On the other hand when we create JsonObject from String - parsing exceptions might happen and these are reported as JsonConversionException. The same might happen if we try to create JSON from simple type (String, Number etc). 

Best regards
Sebastian
Sure, I'll fix it right away. I didn't know about this approach.