Update setup.py to explicitly include cryptography (#90, #91)

setup.py

@@ -31,6 +31,7 @@
         'requests-oauthlib>=0.3.0',
         'python-dateutil>=2.1',
         'PyJWT==1.4.0', # This is required as part of oauthlib but doesn't seem to get included sometimes.
+        'cryptography==1.3.1', # As above, but fixes issue with missing module imports not picked up for some reason.
     ],
     tests_require=[
         'mock',