Implement a CFSSL issuer plugin (#452)

* Implement CFSSL issuer plugin

Implement a Lemur plugin for generating certificates from the open
source certificate authority CFSSL
(https://github.com/cloudflare/cfssl). The plugin interacts with CFSSL
through the CFSSL REST API. The CFSSL configuration is defined in the
lemur.conf.py property file using property names prefixed with "CFSSL_".

* Update documentation to include CFSSL plugin

docs/developer/internals/lemur.plugins.lemur_cfssl.rst

@@ -0,0 +1,20 @@
+lemur_cfssl Package
+===================
+
+:mod:`lemur_cfssl` Package
+--------------------------
+
+.. automodule:: lemur.plugins.lemur_cfssl
+    :noindex:
+    :members:
+    :undoc-members:
+    :show-inheritance:
+
+:mod:`plugin` Module
+--------------------
+
+.. automodule:: lemur.plugins.lemur_cfssl.plugin
+    :noindex:
+    :members:
+    :undoc-members:
+    :show-inheritance:

docs/developer/internals/lemur.plugins.rst

@@ -27,5 +27,6 @@ Subpackages
     lemur.plugins.base
     lemur.plugins.bases
     lemur.plugins.lemur_aws
+    lemur.plugins.lemur_cfssl
     lemur.plugins.lemur_email
     lemur.plugins.lemur_verisign