Modify inapp is_purchaseable logic (bug 1058953)

- Simulated products are always purchasable
- Stub products are always simulated
- Added tests
this test is unrealistic, ``webapp`` would always be None. I don't think you need this test though because it's a duplicate of https://github.com/JaredKerim-Mozilla/zamboni/blob/1058953/mkt/webpay/tests/test_views.py#L158