Added Gentoo detection, logo, feature detection, and documentation.

This commit adds explicit support for Gentoo Linux thanks to Neowin user
FizzyWidget. It is an enhanced version of the patch he submitted earlier today.
In short, it adds complete support for Gentoo as expected.

Unlike support for other distributions, Gentoo requires a little more than
just name and logo detection. Consequently this commit also adds support for
the Gentoo hostname scheme (which may be used by other distributions as well),
scrot feature detection, and Gentoo-specific dependency documentation.