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

@tjfontaine Thoughts on how we want to proceed and what solution we want to use?