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.

As above, nose will hide this output if the test passes.