Added Excerpt and formatting. Appears on index.

Issue Reference: #12

Hopefully I did this right! :) 

Jekyll will use the first block of text until it hits the excerpt_separator as the post.excerpt text. Since our post.excerpt is wrapped in a \<p>\</p> and the post text itself contain these tags, we're getting extras and they are being printed to the screen. My solution was to use the excerpt: override in the YAML Front-matter.

I have tested this locally, but when I initially worked on the fix, I cloned the repo instead of forking it. I don't know my way around Git well enough to figure out how to fix it, so I just copied my changes into a forked copy of the repo and issued a pull request. Please let me know if you run into any issues. Thank you!

Solution: http://jekyllrb.com/docs/posts/