ehnc Featured projects section

Based on newest designs:

https://18f.slack.com/files/ericronne/F02TL5AM4/peacecorps-donationlanding-11-04-14.png

Created a background image class with a centered one for this use case
and a bordered class with white modifier.

Other changes:
- Added left right paddings to all headings, need to fix up.
- Added no bottom margin class for text. Useful for many headings.
Merge pull request #67 from msecret/enhc-projects_section

ehnc Featured projects section