Wallet encryption

[jeanpaul]

Currently, the wallet is not encrypted, which might be fine for small amounts, but it would be better to encrypt the wallet and unlock it when needed (i.e., when transferring funds to another wallet).

I'm thinking about two options for getting the passphrase:

1. When starting the bot, ask for the key with the getpass module
2. Get the passphrase from the command line

The second option is a little less secure, because (without proper precautions), the passphrase is visible from the process list. The first option might be a little inconvenient when starting the bot using a script.

For now, I think I'll settle for the more secure option, and maybe add the command-line option for the daredevils.