Use existing validation hooks (avoid forcing users to hack core Comm Events code)

Hey man - awesome stuff!

This is a rough set of changes you might want to put your own stamp on (if you like them at all), but basically what I looked at here was:

* Avoiding the need for users to modify core Community Events code by making sure the two captcha fields:
  * Are not wiped out by the submission scrubber
  * Are validated using the same mechanism that applies to all the other fields
  * ... this also means a meaningful error message should be returned if the captcha check fails
* I also added a check on plugins_loaded so as to avoid fatals if (for instance) Really Simple Captcha isn't installed

Let me know your thoughts!