SimpleWallet gets support for subaddresses

Command-line wallet for Talleo will soon have support for using subaddresses. Wallet daemon has already had support for them since launch.

Subaddresses are used by online wallets, including webwallet, exchanges and bots to separate funds of different using. Subwallets are more secure alternative to using payment ID as subwallets don't reveal themselves in blockchain explorer.

Subaddresses use same private view key as the primary address of wallet container, but use separate private spend key, so in normal conditions individual users can't access or spend funds that belong to other subwallets, but can decode addresses that belong to same wallet container.

Main reason for adding support for subaddresses to SimpleWallet is easy recovering of funds from multiple subwallet addresses if wallet file needs to be restored from mnemonic seed or private keys.

We will eventually plan to move some subwallet related auxiliary functions from wallet frontends to the backend code, so the functions don't need to be duplicated. These new API methods will include for example filtering incoming transactions by wallet address instead of private view key.