If the Unix `MODE` is set, the choice of configuration changes:

* `MODE=crunch` (or nothing): use static filesystem via crunch
* `MODE=fat`: use FAT and block device (run ./make-fat-images.sh)
Merge pull request #57 from avsm/support-fat

Support a choice of either Crunch or FAT
install: note that OPAM 1.1 or greater is required
Install: refresh instructions to reflect that we need either 4.01.0 or 4.00.1
install: tweak
tutorial: refresh hello world in preparation for mirage-www tutorial
install: note that at least mirage 1.0 should be used, not the beta versions

closes mirage/mirage#146