Fix issue while purchasing already owned items.

The IabHelper invoke onIabPurchaseFinished with null purchase while
purchasing already owned items, fix it.
Implement consumeAsync function.
Merge pull request #1 from HolymarsHsieh/FixAlreadyOwnedItem

Fix issue while purchasing already owned items.
Merge pull request #2 from HolymarsHsieh/ConsumeAsync

Implement consumeAsync function.
Fetching items details from google play
Allowing empty signatures on debug builds
Allowing empty signatures for test purchases only when you specifically ask for it in StoreConfig
fix issue #194, plus preemptive fix for big sku queries
limit store dep to billing
Merge pull request #3 from soomla/fixSkuDetailsChunked

first part of fix issue #194, plus preemptive fix for big sku queries
change comment
change comment
remove whitespace
Merge pull request #5 from soomla/weakerSoomStoreDep

SKUs query result improvement
Merge from upstream