Add monochrome status bar icons (read and unread)

Consistent with Apple's OSX Human Interface Guidelines and #169 the modern OSX status bar icon should be monochrome (currently Vienna is orange where there are unread items).

https://developer.apple.com/library/mac/documentation/userexperience/conceptual/applehiguidelines/IconsImages/IconsImages.html

Icons themselves are from Font Awesome by Dave Gandy - http://fontawesome.io/, used under a SIL Open Font License (OFL) 1.1 license. This is a permissive license compatible with Apache License.