Move VerifySignature from wallet.Backend to wallet.Sig

[manoranjith]

Location

wallet, ethereum/wallet, sim/wallet

Problem

Previously, VerifySignature function is defined on the wallet backend. This looked fine, because until recently, wallet.Sig was defined as a byte array. But, now it has been redefined as an interface with each wallet implementation implementing the wallet.Sig interface (see #292). Now, it looks odd to use the wallet.Backend.VerifySignature method when Sig itself is defined as a separate type.

Proposal

Move the VerifySignature method from wallet.Backend to wallet.Sig interface.

[manoranjith]

@matthiasgeihs