Add scrypt+sha512 key derivation method.

Disabled by default. You can use -derivationmethod option to choose which method will be used for wallet encryption keys generation.

Possible values:

* sha512 - default, compatible with bitcoin;
* scrypt+sha512 - selects scrypt derivation function.

Please note that this option only allows you to select default method. If you are already using encrypted wallet then suitable derivation method will be selected automatically.