simplewallet command to change seed language: "set seed language"

Add wallet2::verify_password method
Allows wallet password to be verified without changing wallet state.

Add simple_wallet::seed_set_language method
Add simple_wallet::set_variable method to provide top-level "set" command and support "set seed language" command.


simplewallet usage:  set seed language

This is for deterministic wallets.

1. Prompt  for wallet password so wallet can be updated with new language. Password is verified only, not updated.
2. Prompt for seed language.
3. Wallet updated with specified seed language.