Don't use null hash for genesis UxOut.SrcTransaction

The null hash is used for the genesis block's unspent output SrcTransaction, instead of the genesis transaction's hash. This affects the UxID of the genesis unspent outputs.

The null hash is not necessary to use here, and some special cases in the code could be removed by using the genesis transaction's hash to calculate the UxID.

The locations where this is special cased are:

- visor.NewReadableTransaction
- coin.CreateUnspents
- coin.CreateUnspent
- cmd/skycoin/skycoin.go InitTransaction (the hardcoded hash `"043836eb6f29aaeb8b9bfce847e07c159c72b25ae17d291f32125e7f1912e2a0"`)

However, this cannot be changed without a hardfork / blockchain reset.

Furthermore, any fiber coins would need to maintain the null hash behavior or else do a hardfork themselves. Consider providing an easy way for them to maintain compatibility when upgrading to newer releases.