Use "npm cache add" for downloading plugins

Plugins were downloaded using the request library. Changing the registry.js
code to use "npm cache add" as the main fetching mechanism. This results in
considerably less code to maintain.

Npm's default proxy settings are used automatically (unless overridden in
~/.plugman/config).

Platform fetching done by cordova/fetch should eventually use npm as well, but
this will require publishing the platforms as npm package tarballs.