BitcoinJKit set bitcoinj wallet to autosave

without wallet.autosave = true, wallets will not get stored after incoming tx, etc.
autosave is a standard behavior and should therefore be enabled.

Signed-off-by: Jonas Schnelli <jonas.schnelli@include7.ch>
Merge pull request #2 from jonasschnelli/bitcoinj_autosave

BitcoinJKit set bitcoinj wallet to autosave