add timestamp to bp_key

timestamp will be needed for SPV mode "fast catchup" blockchain load.

bitcoinj/Multibit does it the same way. Check:
http://bitcoinj.googlecode.com/git-history/3d6691c82a09f7a072f25766720abf0a6d242ef0/tools/src/main/java/com/google/bitcoin/tools/WalletTool.java