blog post teaser image fix + front layout refinement

This pull request will fix the display of blog post teasers on the front page and on the main blog page, only displaying a teaser image if the value for a teaser image is set in the post's front matter. This also incorporates template changes per https://github.com/opentechinstitute/commotion-docs/issues/15

To test, pull the issue-99 branch, build & serve using a local jekyll server, and confirm that "broken" images do not appear in various browsers on the front page / and on the main blog page /blog

To test issue 15, confirm that the front page displays properly using the new layout code in _layouts/front.html