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 I split out a module file `tools/configure.d/nodedownload.py` ( kept general in case there's more factoring done on `configure` ?  ) - let me know how that looks.

@trevnorris seems like if it is on by default we might as well auto download, it seems the most generally user-friendly.   You could also include a copy of icu .zip in node's .tgz which would save folks a step.