Provide another implementation of the types.Signer interface for signing on the Ledger Nano S.

https://github.com/tendermint/tendermint/blob/master/types/priv_validator.go#L60

Note it must be stateful and satisfy the same logic as our default signer (https://github.com/tendermint/tendermint/blob/master/types/priv_validator.go#L206) to prevent double signing!