Packaged for Gentoo

I've packaged Rippled for Gentoo, ./Build/Gentoo can be used as an overlay for portage.
Add a line like this to /etc/portage/make.conf to use it from there.
```
PORTDIR_OVERLAY=".../rippled/Build/Gentoo"
```
The same package is already available in the 'bitcoin' overlay.
```bash
 USE="git" emerge -uN layman
 echo "source /var/lib/layman/make.conf" >> /etc/portage/make.conf
 layman -S
 layman -a bitcoin
 emerge -avu rippled
```
Its been there since https://gitorious.org/bitcoin/gentoo/commit/9a0fd40935b49d3d48a92e49a8461eeec80bfc35

You may notice that there is no init script, just a systemd unit, and that perhaps rippled version out of date. I packaged this for someone else a bit ago and haven't had a need to update it.

I don't actually use Gentoo anymore, but Nixos, packaged here https://github.com/NixOS/nixpkgs/commit/6c77690b2898aa8a12184ca8f3c113a11c957e50
And binaries that have been built by the Nixos build farm are available here http://hydra.nixos.org/build/10082228 http://hydra.nixos.org/build/10084031

rJzEw3cg8pZnGzf5EQcehBfuJGFUywo1S3 :)