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

We can do that also. Seems like download-all or download-none are the
highest running cases: the "just make it work" or "don't touch the network"
model. Want to open an issue against joyent/node for this? (I'm away from
keyboard)