Move to RFC7159 (closes #90)

As mentioned in #90, the restriction to object or array root elements has been dropped with the updated JSON RFC ([RFC7159](http://www.ietf.org/rfc/rfc7159)).  This pull-request removes the restriction from the RapidJSON implementation.

The unit tests are updated to check the new behaviour.

@thebusytypist: Could you please review the changes to the iterative parser (c9f2715)?