Add header icon, and tags display

Hi,

I used your Jekyll theme to create my GitHub blog, and modified it to suite my needs:

 - My logo in the header.
 - Tags display and browsing with no plugin.

**Changes:**

 - Add a 50x50 logo in the header using your logo in `public/` folder.
 - Add page `Tags.html`, which lists all site tags and their related posts.
 - Display post tags left align under their title.
 - Display post excerpt and their tags in `index.html`.

I tried to stick the most I can to your original design, although I am not a designer. So I hope you'll like it and that it will be usefull for others.

Thank you for your work,
Camille