Add basic staking derivatives as CW20 token contracts

[ethanfrey]

This starts with the sample from cosmwasm but cleans it up and makes it fit the cw20 interface.

We still need to iterate on this one a production ready version, but this can serve as a demo and allows contract composition. (See #59 for future improvements)

• Copy original code (and remove transfer functionality)
• Pass all local tests
• Basic cleanup
• Extend cw20 token contract