no need for navcontroller for email popover

great adaptation of askforappreviews !

I just added some code that prevents the need for setting up the nav controller to use the email, it just pops the email over the top of everything.  this way you can just follow the setup instructions that you provided and it works without any custom navcontroller setup.

I also adjusted the readme just slightly to highlight that you need the messageUI framework.  I think the quotes were just slightly off so I kept missing the fact that i needed it.

don't merge the pod spec - i pointed it to my implementation