Add Cooper Hewitt font family

Fantastic font sponsored by the Smithsonian design museum of the same name. Would be a great addition to Google Fonts.

Licensed under the SIL Open Font License and can be downloaded at http://www.cooperhewitt.org/open-source-at-cooper-hewitt/cooper-hewitt-the-typeface-by-chester-jenkins/

A review of the font: http://typographica.org/typeface-reviews/cooper-hewitt-the-typeface/