Add AutRar constants (to JsonMessage).
Implement operator<< to print JsonMessages to streams.
Refactor Json handling of the messages: Use JsonCpp library, delegate json parsing and serializing to it.
Each message type has it's specific fields and get/setters, and implements pack/unpack methods to initialize these fields after parsing and to sync them back to the Json tree before serializing.
Rewrite parsing to use JsonCpp and to handle RarContainer format (not fully supported, parses only the 1. RarItem).

Serializing is now handled fully by JsonMessage base class.

Add lastErrorMessage to PlatformSpecifics.