Added package.js support for themes (and plugins)

Closes #2081
* Amended require-tree to populate availableThemes and availablePlugins to use full file names (`basename.ext`) as keys instead of just basename. This way `image.jpg`, `image.png`, `image.gif` won't overwrite the `image` key.
* Amended require-tree to allow package.json file parsing to return the contents of the file as json on the `package.json` key.
* 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.
* Added error and warning messages for package.json file parse errors and misses
Merge pull request #2156 from javorszky/iss2081

Added package.js support for themes (and plugins)