Rename config var RECAPTCHA_PRIV_KEY → RECAPTCHA_SEC_KEY.
Add docstring to HTTPServer.CaptchaProtectedResource.getClientIP().
Fix docstring for HTTPServer.GimpCaptchaProtectedResource.checkSolution().

It looks like the docstring was aoriginally copied from
HTTPServer.ReCaptchaProtectedResource.checkSolution(), as it says, "The
client's IP address is not sent to the ReCaptcha server..." and never
rewritten.
Tweak docstrings for two ReCaptchaProtectedResource methods.

 * CHANGE getRemoteIP() and checkSolution() docstring to make the Sphinx
   output be formatted more nicely.
Clean up HTTPServer.addWebServer() docstring.
Add class inheritance overview to bridgedb.captcha module docstring.
Simplify GimpCaptchaKeyError and ReCaptchaKeyError into CaptchaKeyError.

 * REMOVE exception class captcha.GimpCaptchaKeyError.
 * REMOVE exception class captcha.ReCaptchaKeyError.
 * ADD exception class captcha.CaptchaKeyError.
Add a docstring to captcha.Captcha.__init__().
Make publicKey and secretKey attributes of the ICaptcha interface.

Both the reCaptcha and GimpCaptcha subclasses of the generic base
implementation, bridgedb.captcha.Captcha, of the ICaptcha interface have
publicKey and secretKey attributes. In fact, it would be hard to
generate a secret CAPTCHA challenge_string which was retrievable later
if there weren't any keypairs involved. Ergo, the keypair should be part
of the interface specification.
Fix captcha.Captcha.get() implementation.

It's not supposed to return anything. It's only supposed to set the
Captcha.image and Captcha.challenge attributes.
Simplify Captcha and CaptchaProtectedResource keypair attributes.

All *Captcha and *CaptchaProtectedResource classes and subclasses (as
well as their interface specifications) now have `publicKey` and
`secretKey` attributes.

For the GimpCaptcha class, this involved switching the ordering of the
parameters for the initialisation method from:

  GimpCaptcha(secretKey, publicKey, ...)

to:

  GimpCaptcha(publicKey, secretKey, ...)

Make sure that the intialisations of all of these classes take the
public key *first*. YOU HAVE BEEN WARNED.
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