adding list of fellows as YAML to each city, relates to #5

This commit places a new YAML section in each city called `fellows` and lists out each fellow for the respective cities. Example:

```
fellows:
  -
    name: "Ben Golder"
    url: "http://www.codeforamerica.org/people/ben-golder/"
    image: "http://www.codeforamerica.org/media/images/people/ben-golder.jpg"
  -
    name: "Emma Smithayer"
    url: "http://www.codeforamerica.org/people/emma-smithayer/"
    image: "http://www.codeforamerica.org/media/images/people/emma-smithayer.jpg"
  -
    name: "Sam Matthews"
    url: "http://www.codeforamerica.org/people/sam-matthews/"
    image: "http://www.codeforamerica.org/media/images/people/sam-matthews.jpg"
```

The commit adds a new `_include` file called `fellows.html` that is called within `_layouts/page.html` (and can be re-ordered if necessary!). To spice things up a bit, I also added a new stylesheet `custom.css` within the `public` directory so we can write our own CSS without bashing the theme files. This only used to style the fellows list currently. Let me know if you'd like to see it elsewhere.

That's about it! We could add descriptions, but I think linking to our pages at codeforamerica.org should do it for now. Going to create this PR and let someone else merge after confirming/testing. Cheers! :beers: