added a helloblock integration test for OP_RETURN

I've created an integration test that creates a random message in a OP_RETURN, propagates the transaction, and then checks to see that it went through and decodes properly.

It uses a random 5 character string for the test and you can see them here: http://testnet.coinsecrets.org/

Lemme know what you think! I've tried to adhere to the code style as close as possible.
@weilu maybe it would be nicer to allow users to check classification for only their type (that is, expose `scripts.isNullDataOutput`)...