Automatic Javascript installation on prepare

Uses <js-module> tags to automatically inject code into the
platform-specific cordova.js.

Also the CLI now uses <asset> tags properly, rather than blindly copying
the entire www/ directory. This copying is done on prepare, since we're
trying to avoid touching the user's top-level www/.
plugin add/rm no longer change top-level www
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/cordova-cli
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/cordova-cli