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

@misterdjules It is support for including a stripped-down small ICU source in the node repo. This enables `--with-intl=small-icu` to work without downloading anything, and was requested at one time. It can be removed or at least documented:
* `icu_small_tag` - a small file which denotes that a subdirectory is a copy of the `icu-small` dir
* `icu_small_path` - if present, this is a directory which contains the "icu-small" source.