Add Go code samples/tutorials for Lending Protocol

[oeggert]

Code

• lending-setup: Waiting on Counterparty Signature helper function.
• create-loan-broker
• cover-clawback
• cover-deposit-and-withdraw
• create-loan: Waiting on Counterparty Signature helper function.
• loan-manage: Need to update flag parsing to use Contain() utility function.
• loan-pay

Add to Tutorials

• Create a Loan Broker
• Claw Back First-Loss Capital
• Deposit and Withdraw First-Loss Capital
• Create a Loan
• Manage a Loan
• Pay Off a Loan