[READY] Implement tags overview

![screen shot 2014-08-27 at 16 06 39](https://cloud.githubusercontent.com/assets/642544/4060879/7f4d9ba4-2df3-11e4-92ed-adb32dd9dfed.png)

In this overview you can see the hours spent on a certain tag. You see the hours on that tag per user and per project. Also all the entries for that tag are listed, your entries in there can be edited. 

I also renamed the javascript charts file from project to charts since the charts are used in multiple places. I also moved some css to application because it was used in more places. Last of all i updated the annotation.
I would memoize this because right now it'll be executed twice

    @tags ||= Tag.find_by_slug(params[:id])