Added package.js support for themes (and plugins)

Closes #2081
* Amended require-tree to allow package.json file parsing to return the contents of the file as json on the `package` key. Caveat: any other file / directory with extension other than .json will likely break this behaviour
* settings api populates theme data package if it exists. Otherwise it assigns `false` to it
* `general.hbs` (salute) was reworked to if there is the package key on the theme is not false, it will use the `name` and `version` keys of that. You can break it by not having a `name` or `version` in the package.json file.