Allow getaddednodeinfo result to be string slice.

The getaddednode command dns flag alters the output.  It is a JSON object
when true and a slice of strings containing the addresses when false.
Note there is a bug in bitcoind as of version 0.8.6 which returns the
addresses as a JSON object with duplicate keys.  This has been reported as
issue 3581 on the bitcoind issue tracker.

This commit allows the result for getaddednodeinfo to be either the JSON
object or the string slice.