Add cw20-ics20 contract

[ethanfrey]

Requires #225

Use the new IBC entry points to create a contract that accepts cw20 tokens and sends them to a remote chain via ics20. Must accept the tokens it issued and return them to the original contract.

Accepting remote tokens (issued on other chain) and creating a new cw20-base contract to handle them is out-of-scope here and can be done in a future issue if desired (but maybe they should just connect to transfer?