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
Merge pull request #59 from avsm/master

updates to install/hello world