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.
Merge pull request #200

cfc8c55 Add simple_wallet::seed_set_language method (warptangent)
26b87df Add wallet2::verify_password method (warptangent)