Added a generic Sitemap, XSL, and robots.txt

I added a generic sitemap and robots.txt.

In `robots.txt` update

```
Sitemap: http://YOUR_DOMAIN.TLD/sitemap.xml
```

Posts are automatically added to the sitemap. Pages must be explicitly added by putting the following in the Front Matter.

```
---
sitemap:
    priority: 0.7
    changefreq: monthly
    lastmod: 2013-03-29T12:49:30-05:00
---
```

__References/Sources:__ 
+ http://davidensinger.com/2013/03/generating-a-sitemap-in-jekyll-without-a-plugin/
+ http://yoast.com/xsl-stylesheet-xml-sitemap/