multi-test: Add migrate support

[ethanfrey]

Builds on #350 (storing the contract admin)

• Add another optional method migrate to the contracts
• If we call WasmMsg::Migrate, verify the caller is the admin (or error)
  • switch the code_id for the contract
  • execute migrate on the new code_id
  • execute any Response from migrate
  • if this fails, revert all changes (including code_id change)
• Expose top-level helper in App for easier testing/usage. But contracts can now migrate other contracts