Closes #260 by filtering html from post excerpts

- The issue was that post excerpts would look inconsistent depending on
  whether they used the <!-- more --> tag or a manual excerpt
- This commit filters whatever html may or may not be in an excerpt and
  wraps the excerpt in a <p> tag. The consequence is that all excerpts,
  even if they span multiple lines will be displayed on the homepage or an
  archive in a single `<p>`
Closes #258, touches up some post excerpts

- Some of the post excerpts were a little light on content so I moved the
  <!-- more --> around a bit.
- Rewrites the full blog index to use `excerpt` instead of `description`
  for presentation consistency.
Merge pull request #262 from 18F/258-patch

Closes #258, touches up some post excerpts