Add Connection.get_certificate method (#733)

This makes it possible to retrieve the local certificate (if any)
for a Connection.

An example where this is useful is when negotiating a DTLS-SRTP
connection, the fingerprint of the local certificate needs to be
communicated to the remote party out-of-band via SDP.
diff --git a/src/OpenSSL/SSL.py b/src/OpenSSL/SSL.py
index 4c708ba..e3eddae 100644
--- a/src/OpenSSL/SSL.py
+++ b/src/OpenSSL/SSL.py
@@ -2176,6 +2176,18 @@
         """
         return self._socket.shutdown(*args, **kwargs)
 
+    def get_certificate(self):
+        """
+        Retrieve the local certificate (if any)
+
+        :return: The local certificate
+        """
+        cert = _lib.SSL_get_certificate(self._ssl)
+        if cert != _ffi.NULL:
+            _lib.X509_up_ref(cert)
+            return X509._from_raw_x509_ptr(cert)
+        return None
+
     def get_peer_certificate(self):
         """
         Retrieve the other side's certificate (if any)