Add multisig support for data in transactions (as long as data.length is not empty)

[bencxr]

Currently data transactions with 0 value are always sent out using only a single owner. This is dangerous as it can apply to other tokens (such as DAO) where there is token transfer, but no ETH is transferred.

This change will trigger multisig (multiple owners) required for non-eth transfers (or any transaction including data). We should do this because the outcome is not possible to identify in this contract, and so it is safer to require multiple signers.

I'm working in another repo where I've added tests for the wallet. You can view the check in and test here: https://github.com/BitGo/eth-multisig-v2/commit/013bc613c77a92da1fc7c403151d54fb6d226e92

[bencxr]

Pinging @chriseth, any thoughts on this?

[chriseth]

I would prefer a full rewrite of the wallet contract. I have seen a pretty simple rewrite, but I'm not sure if it is published yet.