Initial implementation of offer wall support.
The metadata is supplied in the storefront object (via theme.json).  Opening the offer wall entails calling the native API, which in turn calls a JS API function with options that are used to open the offer wall in an iframe
Add getOfferWalls API
Add TODOs
- Add hooks object on store model object
- Add consts
- Add theme object for encapsulating the theme JSON
Delete hooks from toJSON
Change theme object to "asset manager"
DRY direct use of modelAssets object, move all references into the asset manager
Add a few more API functions to economy models
Fix getName
Add string utils, pnotify and handleMessage API
Move native API modules one next to the other
Organize requirejs path configuration and file locations
Change hook initiation API call to pass a JSON object
Change hook API to pass provider as first argument and options as second
Add hooks manager and hooks mixin
Sponsorpay hook: item ID specific
Add the template module path with another call to require.config
Add API for checking offer wall support and for getting the Sponsorpay hook
Upgrade to underscore 1.5.2
Allow saving objects in model assets.  If an object exists, it is assumed that it has at least one attribute named `default`.