Automatically download ICU, enable Intl `--with-intl=small-icu`by default

Background: see joyent#7676 of course

* [x] decide which variant is on by default
* [x] make it on by default
* [x] test more failure cases of downloading
* [x] add md5 sum
* [x] test more platforms

@rvagg didn't see any precedents here. It certainly could be factored out into a separate utility or at least imported function.