Fix and update gentoo-latest-x86_64-experimental

The existing gentoo-latest-x86_64-experimental template is dated and broken. I used the beautiful https://raw.github.com/jedi4ever/veewee/master/templates/funtoo-latest-x86_64/postinstall.sh as an example and added a few gentoo-specific tweaks to make it work:

* Gentoo no longer host md5 checksums for their isos (see http://distfiles.gentoo.org/releases/amd64/autobuilds/20130110/install-amd64-minimal-20130110.iso.DIGESTS), so I had to remove `:iso_md5`
* Fixed kernel version to 3.6.11 since `boot-update` is only available in Funtoo
* Added CONFIG_RTC to the kernel config to make `hwclock` not fail
* Made locale configurable to enable UTF-8 from the start
* Used the stock ruby 1.9 ebuild - I'd rather have the end user to make a choice between rvm, rbenv, chruby etc.

The resulting box is 857.5Mb and includes all the compiler infrastructure as well as kernel source.