replace lines with call to recently added print_seed()
indentation
Extract check for deterministic keys to wallet2::is_deterministic()

It's cleaner for wallet2.cpp and it also allows deterministic check by
simplewallet.cpp.
Checking and handling for deterministic vs non-deterministic wallet

simple_wallet::seed()

 - Check that wallet is deterministic.

simple_wallet::new_wallet()

 - Prompt for seed language only if it's a non-deterministic wallet,
   along with previous conditions.

simple_wallet::open_wallet()

 - Fixed check for deterministic wallet (flag based on command line
   non-deterministic argument was used before, but it's inapplicable to
   opening an existing wallet).

 - As with deterministic wallet, non-deterministic also included to be
   rewritten to new JSON format file. That's what's done for newly
   generated non-deterministic wallets, so old versions should be
   updated to same format.
wallet JSON update for non-deterministic wallet data

wallet2::store_keys() and wallet2::load_keys() should only use the JSON
attribute "seed_language" when applicable. That is only for
deterministic wallets.

 - store_keys()  don't add JSON attribute "seed_language" if
   seed_language is empty

 - load_keys()  don't call set_seed_language if JSON attribute
   "seed_language" not present
Merge pull request #196

f9822c4 wallet JSON update for non-deterministic wallet data (warptangent)
4c6230d Checking and handling for deterministic vs non-deterministic wallet (warptangent)
1beedb9 Extract check for deterministic keys to wallet2::is_deterministic() (warptangent)
359ede3 indentation (warptangent)
2290eff replace lines with call to recently added print_seed() (warptangent)