Unify local and external keys in keybase interface

[cwgoes]

Rework PrivKey, Keybase, and Info to work with both locally stored and external keys (offline, HW wallets, etc).

This involves the following API changes:

• Return errors on privkey.Sign() and privkey.PubKey(), as we may be querying an external datastore
• Add methods to the Keybase interface to track Ledger HD ed25519 keys and offline keys, so that signing by name automatically looks up the right key and prompts the user for a password / communicates with the Ledger / prompts the user to sign offline, as appropriate.

[cwgoes]

To-do:

• [x] Add way to track a watch-only key by pubkey
• [x] Correctly switch on .Sign() based on keytype
• [x] Add testcases
• [x] Test downstream in Cosmos SDK

[cwgoes]

@Liamsi Thoughts on the API changes?

[liamsi]

The interface changes look good to me! Totally in favour of sign returning an error. I also like changing Info to an interface. I might change the Create to CreateMneomnicKey or simply CreateMnemonic (as the key is clear because of the context/package name/iface name) in https://github.com/tendermint/go-crypto/pull/118