Add Python 3 support for FacebookAuthorization.parse_signed_data

`json.loads` was expecting a string, but in python 3 `base64decode()` return bytes and that is why it was bugged. We fix this by making sure the decoded payload data is in string and that `hmac.new()` is provided with arguments in bytes. `open_facebook.utils.smart_str` will do that job correctly in python 2 and 3.

We also use `hmac.compare_digest()` which is the preferred way to compare those kinds of data to prevent timing analysis. Read more about this on python docs. Though this is only available from python 2.7.7.

Fixes #491.