Remove recaptcha in favor of SpamSniffer

Added SpamSniffer module and spam_prevention_field helper.  This module and helper add a honeypot, hidden field to the form. If it's filled in, assume it's a spam bot.
Non-functional code cleanup

* Removing some unnecessary comments
* Removing some extra "self"s and "return"s
Set Listing's AdStatus as a before_create

Instead of setting it in the controller.
Move "approved?" functionality from Listing to AdStatus

If the name ever changes from "Approved", it can just be changed in the AdStatus model.
Remove old no-conflict JS from application layout
Render <br> in user signup errors, instead of showing the HTML markup