Added support in Purchases extension for getTitle/description/price.

Updated Purchases extension where the product list is first requested
from the App Store, then only valid purchases are allowed.
- Added Script.purchasesRequestProductInfo which needs a new block in
Stencyl, called "request product info" or " request product data".
- Stencyl's current get title/description/price blocks need to be hooked
up to Script.purchasesGetTitle, Script.purchasesGetDescription,
Script.purchasesGetPrice.
- A new boolean should be added in Stencyl for the existing
Script.purchasesAreInitialized. Maybe call it "can buy" or "can
purchase" instead.
- Added new event PURCHASE_PRODUCTS_VERIFIED which is called when
purchasesRequestProductInfo succeeds. Maybe call it "product info is
received" or "product data is received".
How to use:
1. When your game starts call Script.purchasesRequestProductInfo with a
productID list.
2. Then in your store call Script.purchasesGetTitle,
Script.purchasesGetDescription, Script.purchasesGetPrice for each
productID.
3. If purchases are disabled on the device, or if
purchasesRequestProductInfo failed, they will be blocked. Call
Script.purchasesAreInitialized to check the status.