wallet: fix custom scrypt parameter usage

[ixje]

1. scrypt parameters for the Account instance were not taken into account when calling sign(). Therefore it would always assume the default scrypt parameter. This caused failures in some unittests that used custom scrypt params
2. when importing a multi-signature account the wallet will try to augment that account with the key material of the related accounts so it can be used for signing. This augmentation did not copy the scrypt parameters which could lead to the same issue as 1.

[coveralls]

coverage: 80.751% (-0.03%) from 80.779% when pulling 98f140c on fix-scrypt-params into f607693 on master.