name_history is unimplemented

[JeremyRand]

The name_history RPC method from Namecoin Core needs to be implemented in Electrum-NMC. I think it could be done something like this:

sh = electrum.names.name_identifier_to_scripthash(b'd/wikileaks')
result = await network.get_history_for_scripthash(sh)

We'd also need to do SPV verification on all of the resulting transactions.