add total plugin count and cumulative rating for all plugins

Alright, I took a stab at this: https://github.com/easydigitaldownloads/wp-tally/issues/7

* I didn't include any styles. I don't have the theme so that can easily be added. 
* I tried to do the cumulative rating math based on ratings (duh)... not plugins. For example, I have 6 plugins but only one has been rated. 
* I have no idea what I'm doing. So if this is a sh*t show, everyone just slowly walk away like it never happened. Also, teach me. :)