Add eth_signTypedData

[rmeissner]

Adds support for eth_signTypedData according to EIP-712

This is required for #515

[rmeissner]

How should conflicts in npm-shrinkwrap be resolved?

[benjamincburns]

Hi @rmeissner! Thanks for the contribution.

I think shrinkwrap conflicts should be handled by making sure that existing versions don't change.

Otherwise I'm a little hesitant about this change, primarily because EIP-712 is still listed as draft status, and I can't find where either geth or Parity support it yet. I see that MetaMask has added support, however - so maybe that's good enough since most web apps will be interacting with the client via them or some other wallet middleware...

[benjamincburns]

As I said on the issue, I'm walking back that comment a bit - I really like the interaction this helps to support and I'd rather help move things forward by adding preliminary support for the RPC call. I'll merge this.