Set CLIENT_VERSION_IS_RELEASE, bump build to 0.8.2.1
More bestblock records in wallets

Write bestblock records in wallets:
* Every 20160 blocks synced, no matter what (before: none during IBD)
* Every 144 blocks after IBD (before: for every block, slow)
* When creating a new wallet
* At shutdown

This should result in far fewer spurious rescans.
Merge pull request #2652 from gavinandresen/version_0.8.2_final

Set CLIENT_VERSION_IS_RELEASE, bump build to 0.8.2.1
Merge pull request #2682 from sipa/morewalletbb

More bestblock records in wallets
doc/README was replaced by README.md