Enable read/extract support for ISO images

Hi Erik,

this is (almost) exactly what I've been looking for: an FFI wrapper for libarchive; the one thing I need is for it to extract ISO's, too. This patch does that.

The one thing that is miisng is the test ISO, since it weighs in at 12 MB. I generated one with

    cd test/
    mkisofs -R -J -o libarchive.iso -root libarchive libarchive

BTW, is it really necessary to use the full libarchive sources for the test archives ? I would imagine a few kB of well-crafted files would achieve the same. For what I need, I'd actually like to have a file with a name > 64 characters in there. Would you accept a PR that cuts down the size of the test archives dramatically ?

I tested my patch on Fedora 19 with libarchive-3.1.2-7.fc19.x86_64 and jRuby 1.7.4 / OpenJDK 1.7.0_60 - everything seems to work great.

thanks for putting this together,
David