Metamask only sign the transaction (and don't broadcast it)

[rdig]

Metamask currently only allows us to boardcast the transaction when signing it. We can't just sign it, and then broadcast it some other way.

This is a limitation for us since this library is an offline wallet (well, with the exception of this anyway...)

This is issue is here to keep track of Metamask's development, and when (or if) it gets implemented, bring the functionality in.

Issue to track: MetaMask/metamask-extension#3475