Expose methods for unlocking and locking keys (wallets)

Currently the only method to unlock a key is calling method Client.authenticate(). This method invokes key verification each time – it's unnecessary, because we again verify key already verified and stored in our super-safe storage. It costs us 2 http requests as well. There's not any method for locking key, but should be. Also I cannot see any property to set the period for Beekeeper session timeout.