Automatically download ICU, enable Intl `--with-intl=small-icu`by 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

@rvagg @trevnorris I guess I could add `import tools.downloader` to configure and then create `<node>/tools/downloader.py` right?  or maybe `import downloader from tools`?

Glad to do this. Just don't know what the tradeoffs are time wise.