Update README.md

Fix #32
Added frbr icons to assets
Added styles for _plus images and added additional imgs
Rewrote _plus images as transparent bg
Updated FRBR images
Updated website icons to be transparent
Resized all content type icons, good to start from a standard size
New header image
Merge pull request #34 from NYULibraries/development-newheader

New header image
Hide BobCat logo for phones
Merge branch 'master' of github.com:NYULibraries/nyulibraries-assets into development-login