captcha 6.x-2.1

A new year, a new release from the CAPTCHA 6.x-2.x branch. Changes since previous release (6.x-2.0):

New features in image CAPTCHA:

• #571344: use multiple fonts in image CAPTCHA.
• Added font previews on admin page
• #644046: image CAPTCHA ignores spaces in the user's response (lower barrier for humans, but not easier for bots)
• added small random rotation to characters in image CAPTCHA

Some minor bug fixes

• #586232: 2nd argument call_user_func_array() should be array
• #586232: call_user_func_array() call problem
• #662398: better update path from 5.x-3.x or 6.x-1.x: change text CAPTCHA (which was removed) to simple math CAPTCHA

And further

• Various translation updates
• updated outdated readme file and added note about caching
• minor code cleanup/reordering and documentation updates