This way IDEs can verify method overrides in subclasses, otherwise these
are flagged as erroneous.
Changed base classes to properly raise NotImplementedError; previously
they would cause "TypeError: exceptions must derive from BaseException"
Also fixed exception handling in sources.service.clean().
Mostly typos, grammar errors and inconsistent indentation in code
examples.
Some errors detected using Topy (https://github.com/intgr/topy), all
changes verified by hand.
* 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
