Support Subject Alternative Name in the OpenSSL backend

Adds only DNS support first
diff --git a/setup.py b/setup.py
index 6376a1f..e0b5738 100644
--- a/setup.py
+++ b/setup.py
@@ -32,6 +32,7 @@
 VECTORS_DEPENDENCY = "cryptography_vectors=={0}".format(about['__version__'])
 
 requirements = [
+    "idna",
     "pyasn1",
     "six>=1.4.1",
     "setuptools"