Add c_bool() function to parse bools in config

The bottle's load_config() method doesn't seem to handle True or False
values, so deal with it via c_bool().
Allow public address reuse

Add a config option (faucet.reuse_addr) that will force the
public_address() property to always return the first address it sees.
This allows for a static donation address (per-run, at least).
Allow configuring public address

If faucet.reuse_addr is configured to a valid address, use that as the
public address to display for donations.