Add python cryptography egg component.

component/python-cryptography/buildout.cfg

+[buildout]
+extends =
+  ../python-cffi/buildout.cfg
+  ../libffi/buildout.cfg
+
+parts =
+  python-cryptography
+
+[python-cryptography-env]
+PKG_CONFIG_PATH=${libffi:location}/lib/pkgconfig
+
+[python-cryptography]
+depends = ${python-cffi:egg}
+recipe = zc.recipe.egg:custom
+egg = cryptography
+include-dirs =
+  ${libffi:location}/lib/libffi-3.0.13/include/
+library-dirs =
+  ${openssl:location}/lib/
+rpath =
+  ${libffi:location}/lib/
+environment = python-cryptography-env