fix nullable productIDs

(cherry picked from commit e53a9cefb86c91d8826c60bb4f60bc5de6fe38d4)
Update README.md

Fix path to `android-store-google-play`
initialize pointer with NULL; dereference pointer to pointer before checking for NULL
fix restore data
(cherry picked from commit a1cb7f0)
fix restore data
(cherry picked from commit a1cb7f0)
update submodules
Merge pull request #88 from herrsorgenfrei/bugfix-1

initialize pointer with NULL; dereference pointer to pointer before chec...
Merge branch 'master' into addBuilders
Merge remote-tracking branch 'origin/master' into addBuilders
add JSON logging
fix onMarketItemsRefreshed for Android
(cherry picked from commit 5e47398)
improve builders