OpenShares transaction validation

[octavonce]

An OpenShares transaction lists a shareholders controlled account in the ledger. The validation logic for an OpenShares transaction must do the following:

• Check that the creator address exists.
• Check that the creator address has the amount listed to be initially sent to the account's balance.
• Check that the creator address has the amount listed to be paid as the transaction's fee.
• If the transaction's fee is paid in the same currency as the one being initially transferred, check that the creator address has the summed amounts.
• Check that the representing stock hash is correctly computed.
• Check that the nonce listed in the transaction is correct.
• Check the validity of the shares object.
• Check the validity of the share map object in relation to the shares object.

[octavonce]

Closed in favor of #20.