Fix notifications

Added "jsonrpc" field and "has_id" flag to &JSON::RPC::Parser::construct_from_req() response to signal that a notification was passed (#5).

Added notification handling as described (and shown in its examples) in current JSON-RPC 2.0 standard when we are sure that the client uses 2.0 (the request has "jsonrpc" field set to "2.0") (#6).