Introduce site favicon assets.

Add rough EatenByGrues-themed favicons for various devices. Favicon
images generated from original artwork via faviconit.com. Polish these
in a future design round.
Link to site favicon assets.

Add favicon links for different device types. Favicon assets and links
generated from original artwork by faviconit.com. See
faviconunit-instructions.txt.
Update build script to copy favicon assets.

Update build script to copy favicon assets directly to the \_site\
folder after the build is complete. Alternately we could add the favicon
assets directly to the \chassis\ folder and Pretzel/Jekyll would copy
them across during the build, but \chassis\ would end up cluttered and
we're in there a lot.