Also
- Removing bare `except:` statements in both the PyCrypto and
OpenSSL verifiers (in the `verify` method).
- Catching the only exception possible in the OpenSSL verifier
(it is `OpenSSL.crypto.Error`).
- Converting the signature to bytes (if not already) in the
OpenSSL verifier.
- Adding a test with both a unicode and bytes signature for
each verifier.
Fixes #201.