Add ESLint + Fix raised issues
Update AndroidIAP
Check that Android error is a string
Fix generation of contributor documentation
Add doc
Prevent finishTransaction on "Purchasing" ones
Update misleading documentation
Refresh receipts & Add info about app
Ignore unregistered products on Android. Closes #78
Prepare for release 3.9.0-beta.4
Prepare release 3.9.1
Merge pull request #65 from j3k0/rb3.9

Release 3.9