Intl: Automatically download ICU if requested. Reduce size. Keep `--with-intl=none` the 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

@srl295  Written this way, it seems that `--with-icu-source` requires `--with-intl=full-icu`. Maybe we could write `--with-intl=[small-icu,full-icu]` instead?