Add support for OCSP stapling. (#580)

* Define the OCSPCallbackHelper.

* Define set_ocsp_status_callback function.

* Reframe this as the "server" helper.

* Add OCSP helper.

* Allow clients to request OCSP

* Some tests for OCSP.

* Don't forget to throw callback errors.

* Add changelog entry for OCSP stapling.

* Require at least cryptography 1.7

* Sorry Flake8, won't happen again.

* How does spelling work?
diff --git a/setup.py b/setup.py
index 4cdf59e..de510f3 100755
--- a/setup.py
+++ b/setup.py
@@ -95,7 +95,7 @@
         package_dir={"": "src"},
         install_requires=[
             # Fix cryptographyMinimum in tox.ini when changing this!
-            "cryptography>=1.6",
+            "cryptography>=1.7",
             "six>=1.5.2"
         ],
     )