Allow guests to use process modules

Hi Ryan, I've been using processwire for my most recent projects -- really loving it. I'm sending a couple pull requests your way, starting with this one; I had to make changes to core for a couple projects. And I'm wondering if I'm either missing something or these are changes that would be helpful to the community. 

I couldn't figure out why you would want to prohibit guests from accessing process modules, isn't it enough to simply not give them permissions to the admin page? 

I created a couple custom modules that inherited from Process and then found out that guests weren't able to access the corresponding pages.