Add support for unicode passwords

[abbot]

Copying from #2, something roughly like:

def mangle(s): return "".join(chr(ord(x) & 0xff) for x in s)

def generate(password, uri):
    """
    generate the pwdhash password for master password and uri or
    domain name.
    """
    realm = extract_domain(uri)
    if password.startswith(_password_prefix):
        password = password[len(_password_prefix):]

    password_hash = b64_hmac_md5(mangle(password), realm)
    size = len(password) + len(_password_prefix)
    nonalphanumeric = len(re.findall(r'\W', password)) != 0

    return apply_constraints(password_hash, size, nonalphanumeric)