Basic multisig contract

[ethanfrey]

Builds on #79

Define a basic, usable multisig

• Define fixed participant set in the beginning
• Each participant may have a different weight
• "Pass" threshold defined on init
• Proposal expiration defined on init
• Any member may create a proposal (&[CosmosMsg]) with an expiration
• Any member may vote once for a proposal.
• If enough yes votes are applied, any member may execute the proposal
• Implement all queries
• Unit test coverage

In addition, specify a standard interface for the multisig, which can be used by more complex implementations.