Make engine hold AccountProvider

[keorn]

Adds a cli --engine-signer ADDRESS (to be used with --password) and parity_setEngineSigner(address, password) RPC.
Now Engine can sign messages whenever it wants without unlocking the account permanently or using --author.

[coveralls]

Coverage increased (+0.03%) to 85.922% when pulling 92d566c on engine-password into ad36743 on master.

[gavofyork]

would it make sense to choose one of setEngineSigner/setConsensusSigner and stick with it throughout?

[keorn]

Sure will change the internals, had to change since --consensus-signer was too long for the current cli setup.

[coveralls]

Coverage increased (+0.001%) to 85.896% when pulling 0a2ec31 on engine-password into ad36743 on master.

[coveralls]

Coverage increased (+0.001%) to 85.896% when pulling b089aa7 on engine-password into ad36743 on master.

[coveralls]

Coverage increased (+0.01%) to 85.905% when pulling 71a9658 on engine-password into ad36743 on master.

[coveralls]

Coverage decreased (-0.03%) to 85.864% when pulling 727ace0 on engine-password into ad36743 on master.

[coveralls]

Coverage increased (+0.02%) to 85.914% when pulling 727ace0 on engine-password into ad36743 on master.