Support JPM addons (bug 1037201)

https://bugzilla.mozilla.org/show_bug.cgi?id=1037201

@kmaglione This might not cover everything. Please let me know if I'm missing anything. Here's what it does:

* Accept the combo of "package.json" and "bootstrap.js" as jetpack addons.
* Validate the `id`, `name` and `version` fields of "package.json"
  * Perhaps `engine` should be validated as well but I'll do that tomorrow.
* All "package,json" errors and "install.rdf" errors are treated as errors, even if both files are present.

Olympia PR to come.

I don't know why, I just moved this code so it could be shared between the install.rdf and package.json validations. I'll file a bug to revisit these once the jpm stuff is live.