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

It's supported, and already landed in v0.12. That's why small icu links
insuch  an odd way. If the "icu dir" option/env var is given, node_i18n.cc
doesn't call setcommondata and thus overriding happens (as icu is actually
linked against the stub data ).