Update blog snippet layout

- Adds `<!--more-->` to all current blog posts so that Jekyll's excerpt function can be used
- Adjusts layout on '<p class="authors">` to normalize author display throughout site and remove extra margin on some pages
- Removes unnecessary `<p>` tag in layout -- was there to make up for the lack of the above, now unnecessary.
- Adjusts news archive layout slightly to match simple blog layout on rest of site.
- Closes #215