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

Is it actually supported? I thought it was [not currently supported](http://bugs.icu-project.org/trac/ticket/10924), but that we planned to fix it later.