Allow importing private keys

[dev0tion]

No description provided.

[sondreb]

May I suggest adding support for sweeping first? Adding support for holding a separate list of private keys can be dangerous and users needs to be fully informed and educated on the difference. They also need full instructions on how to take proper backup after importing. If the user fails to take backup of wallet, their imported funds will be lost.

If you create a seed wallet in Electrum today, it no longer supports importing, only sweeping: "In Electrum 2.0, you cannot import private keys in a wallet that has a seed. You should sweep them instead."

The API for sweeping should allow custom address (so I could potentially sweep and send the funds to anywhere I want), or by default it should take the next unused address, and not the change address, so that it appears as a regular incoming transaction for a user.