remove restore from prepare. Break into its own command

Proposal at https://github.com/cordova/cordova-discuss/pull/54.

New command: `cordova install`

This will fix a lot of issues and weirdness people have with cordova.

cordova install command is very much inline with node/npm workflow.

We can possibly add a warning in cordova-cli that checks if all of your deps are installed and warns you if they are not.