Improve FancyCaptcha resistance to OCR

Tesseract is a popular open source OCR package. Running it on
FancyCaptcha images, with no training or configuration, yielded a 56%
break rate. By restricting the character set, the OCR break rate was
improved to 66%.

So:
* Reduce k, increase wob scale, increase rr fuzz. The net effect of
  these three changes is to more reliably bend the baseline. In the old
  captcha, the baseline would often be bent by chance, but when it
  wasn't bent, it provided a very easy challenge for the OCR engine.
  This reduced the break rate from 66% to around 40%.
* Introduce additive noise, based on a bilinear upscale of a random
  greyscale image. This, combined with the above change, reduces the
  Tesseract break rate to 6%.

Change-Id: I05b5bb6475de9378cd89cce13b1b2f28b32cd405