Merge pull request #8 from matrix-org/server2server_signing

docs/server-server/signing.rst

synapse/api/errors.py

synapse/api/urls.py

synapse/app/homeserver.py

synapse/config/server.py

synapse/crypto/keyclient.py

synapse/crypto/keyring.py

synapse/crypto/keyserver.py

synapse/crypto/resource/__init__.py

synapse/federation/__init__.py

synapse/federation/replication.py

synapse/federation/transport.py

synapse/federation/units.py

synapse/http/client.py

synapse/http/server_key_resource.py

synapse/server.py

synapse/storage/__init__.py

synapse/storage/_base.py

synapse/storage/keys.py

synapse/storage/schema/keys.sql