Remove mkimage-centos.sh in favor of a new rinse-based script...

since febootstrap is fragile and picky.

Febootstrap is officially deprecated by its developers, but besides that is really only useful to create a chroot system of the currently running system (ie, creating a centos-5 container on centos-5).

Rinse is more similar to debootstrap in that it can run on non-yum based platforms.  This is the script I used to create `tianon/centos:5.9` and `tianon/centos:6.4` (from Gentoo, no less) which are on the index currently (and are much smaller than the current official centos image):
```
$ docker images tianon/centos
REPOSITORY          TAG                 ID                  CREATED             SIZE
tianon/centos       5.9                 f642a7164971        2 weeks ago         136 MB (virtual 136 MB)
tianon/centos       6.4                 f06d83c67433        2 weeks ago         126.9 MB (virtual 126.9 MB)
tianon/centos       latest              f06d83c67433        2 weeks ago         126.9 MB (virtual 126.9 MB)
$ docker images centos
REPOSITORY          TAG                 ID                  CREATED             SIZE
centos              6.4                 539c0211cd76        6 months ago        322.1 MB (virtual 322.1 MB)
centos              latest              539c0211cd76        6 months ago        322.1 MB (virtual 322.1 MB)
```

/cc @unclejack @metalivedev