Added dynamic loading of products.

Since this plugin doesn't have knowledge of product ids, we load them whenever we're told to buy them.  We keep track of prior loads to prevent unnecessary loads.
Moved StoreKit initialization to a function.

Initialization is no longer called on deviceready, but rather on the first action (so far, just buying).

Also cleaned up the logging.
Added getSkuDetails.

The loading of all valid products is recorded so they can be purchased later without reloading.  Invalid product ids are logged.
Moved StoreKit initialization to deviceready.
Refactored events and added basic error checking.

Also changed `buy` to delegate to platform-specific implementations.