eth/key: allow for usage of HD wallets

[q9f]

• https://rubygems.org/gems/ethereum_tree
• https://rubygems.org/gems/money-tree

[bdescamps]

Hey q9f do you think hd wallet should be a part of eth.rb gem or money-tree ? To me, i think it could be great to add this in money-tree and have a specialized gem for HDWallet ( Bitcoin / Litecoin / Ethereum...)

[q9f]

@bdescamps money-tree has already HD-wallet support and ethereum-tree is the (unmaintained) ethereum addon for that.

I don't really want to pull in both as a dependency so for now I only have very primitive key management support.