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 Probably a dubious use case, but if someone uses `--with-icu-source=deps/icu`, then it seems that it would not fail gracefully. Maybe we should not delete `icu_full_path` if `icu_full_path == with_icu_source`?