caching, multithreaded resumable downloads

[davidmarkclements]

just pushed this: https://www.npmjs.com/package/install-nw

happy for you to take it and re-use if you think all or any part is appropriate

[shama]

Wow I think all of that is rad. Would you mind sending a PR for it?

[davidmarkclements]

so... it depends how you want to use it,

because of code boundaries install-nw takes this approach:

1. if zip is not in cache, download the zip and store in cache
2. create http server that responds with the cached zip
3. install nw, setting nwjs_urlbase to the temporary server

It also then has to wrangle with the nw package.json file,
(basically we have to make sure that if npm install is run in a folder where nw is already installed, that it doesn't run the postinstall script - because that will revert to default behaviour).

I can wrap nw-install around nw as a PR, OR we can go for a more integrated approach,
e.g. by removing the temporary server dependency, get rid of package.json dance

This second approach would probably lead to lot's of refactoring so I'd suggest that you
lead that one out.