Finish Implementing handleDepositMsg about plasma-mvp-sidechain

[colin-axner]

There are a few things to consider when implementing DepositMsg:

1. I think only Address' corresponding to validators should be able to create/sign this transaction type. In the case of issue #5 we will probably want genesis utxo's to have a value of 0 and be used solely to sign off on depositMsg's. In the future we will need to consider a msg to allow validators to create a new validator utxo for any new validators that enter the chain.
2. If a deposit is made on the root chain, the next block submitted on the child chain needs to be the deposit block. We will need to ensure this functionality on the child chain, but I am not sure exactly where to handle that yet.

Perhaps instead of the above we can maintain a list of approved validators instead of using utxo's, this list would need to be created/set on genesis. Updates to this list would probably require a transaction signed by all validators?

Thoughts?