support for Ed25519 signatures

This increases the versioned dependency on the cryptography module to
2.6, since that is the version that provides the necessary ed25519
functionality.

We also add a "pure" 25519 OpenPGP certificate for testing purposes.

Closes #221, #222, #247

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

setup.py

@@ -22,7 +22,7 @@ with open('README.rst') as readme:
 
 
 _requires = [
-    'cryptography>=2.5',
+    'cryptography>=2.6',
     'pyasn1',
     'six>=1.9.0',
 ]