RPC for importing geth keys

[gavofyork]

Closes #1906
Depends on #1914

Two RPCs:

• personal_listGethAccounts () returns [account0, account1, ...] for each key found in the Geth key store.
• personal_importGethAccounts ([account0, account1, ...]) returns [imported0, imported1, ...] will import each given account into the Parity key store.

Importing Geth keys now done either through an explicit CLI option --import-geth-keys or through the on-ramping in the UI.

[coveralls]

Coverage decreased (-0.2%) to 85.898% when pulling 5ebf668 on rpc-import-geth-keys into 07844d6 on master.

[arkpar]

Intentional?

[arkpar]

import-geth-keys should be a CLI command that does the import and exits

[coveralls]

Coverage decreased (-0.2%) to 85.955% when pulling 5b6e5ba on rpc-import-geth-keys into 07844d6 on master.

[gavofyork]

yeah i agree - i'll put it as a new issue: #1918